The core of the task was to work on the languages each one was learning, but also to do something in group which is always more fun and motivating than doing it alone.

Grammar is OK but this sentence is confusing.
1) "each one" - who are we talking about here? members of a group? You haven't references the group yet, so this lacks context. Furthermore, "one" as a subject isn't often used in English so you have to be careful that it makes sense when you use it.

The solution? I recommend you amend the previous sentence "...challenge with a group of friends." if you can. Then, you can say "The core of the task was to work on the languages each person was learning, ..."

2) "in group" is not a thing. Maybe you're thinking of phrases like "play together in unison". You say "in a group."

3) "which is always more fun and motivating than doing it alone"—this clause is unclear in what it modifies. When I first read this sentence I thought it was modifying "something" i.e. the thing you choose to do had to be fun and motivating. But instead it refers to "to do something in a group", as as additional opinion you have. This is a dangling modifier I believe. https://en.wikipedia.org/wiki/Dangling_modifier

if you put a comma after "group" that will solve your problem. Or parentheses, or an em-dash.

Today, now that the challenge is closing, it’s time to look back and assess what I’ve gained and what areas I need to reinforce.

TOn the positives are that I could catch side, I caught up with my English studies after not having done much in the past 2 years, and to my s. Surpriseingly, I’ve continued to improve in spite of not having worked on my English actively.

this is only 1 positive!

Another thing that makes me close these 30 days with a smile is knowing that I never took the challenge lightly, quite the contrary, every day I spare some time to write something meaningful to me.

Run on sentence!

"Another thing that makes me close these 30 days with a smile" is good but a bit awkward (my English teacher in high school would not approve of stuff like this, it feels a bit like "fluff")

I'm not going to do the edit, but I think it'd be more impactful if you conclude the set of positives with ".. which made me close these 30 days with a smile." It's a nice image, but when you haphazardly put it at the beginning of a sentence where it effectively means "Also," it loses its impact.

In fact, if I had written something quickly just to keep up with the challenge, now I’d be quite dissatisfied with myself.

issue w/ dependent, independent clauses

As for the aspects I need to take more seriously;: Punctuation.

xD

This is also one aspect. Either do:
"The main spect I need to..."
or
"To start with one of the aspects..."

In this regard, mMost of the feedback I got had to do with punctuation.

"in this regard" has no meaning here, and this sentence generally doesn't communicate any new information. But, since it's important, I think you could include it if rewritten like I did.

Hence, this August I’ll put myself to study what relative, dependaent, and independent clauses are.

Sounds good, but "Hence" is pretty formal. You can also just say "so".

However, since I’m looking forward to the C1 certificate, I have to make peace with the idea that English in this regard is different from Spanish.

I'm reminded of high school English all over again! This is the time when many Americans learn phrases like "in this regard" are often just filler phrases that serve little meaning. That might be a key difference between English and Spanish (although I'm only guessing). Effective English writing makes sure that every sentence is meaningful without a lot of filler words.

In this case.. it's obvious you're talking about differences in grammar. And if we kept "in this regard", well, what is "this regard"? The existence of clauses and their grammatical relationship to each other? That seems obvious. It sounds like what you really have to make peace with is that English requires careful consideration of clause construction and placement of phrases, which is a sort of complex topic that you haven't defined.

So that's why I recommend taking "in this regard" out and just referring to the general differences of the languages and having to think about them generally differently.

For example, in Spanish we have more freedom when writing,Spanish punctuation is also more intuitive than regulated.

Ironically this essay is a great case study of improper clause usage :P

"We have more freedom when writing" this fact seems sort of irrelevant. What is freedom? Are we ONLY talking about clauses right now, or grammar more broadly? I recommend just removing it entirely.

Besides we don’t have anything called..., which means we don’t have “run-on sentences”.

Recommend combining with the last one.

That’s why in Spanish one has the freedom to write long sentences, and also to useusing plenty of adjectives to describe the same thing.

", and" implies it's a separate point, but here you're talking exactly about long sentences.

In conclusion,[So I suppose] I’m the one who has to adapt to English grammar and not the other way round.

optional suggestion. Or just remove it.

I would also like to learn another language, consequently thisso these next 30 days I’ll allocate some minutes of my daystime to write in French.

clauses! "consequently" does not function like "so" (which would make sense here)

We don't have a great way to say "allocate minutes of my days." Maybe we would say "spend a few minutes each day."

I have a simple goal: write a 100-word paragraph daily.

Unless the verb is obvious, you gotta write it.

"I have a simple goal: car!" that makes no sense ;)
"I have a simple goal: earn enough to buy a car!" yes!

"I have a simple goal: promotion!" you could probably use this. "get a promotion" is implied.

Luckily I have engraved in my mind, the first time I wrotemy first attempt at writing a paragraph in English, is engraved in my mind. It took me the wholan entire afternoon.

What do you have engraved in your mind?

I would guess: "the grueling experience of taking a whole afternoon to write a paragraph in English".

This is different than the mere objective fact that it took an afternoon. What's engraved is the frustration.

It's not obvious that's what you're referring to until the next sentence. I suggest a restructure above.

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As an aside, with all the words you save eliminating "filler phrases" as I suggest above, you could use them to expand on this experience! It'd be fun to get a better visual of the agonizing anecdote.

I assume that with French I’ll have to go through the same agony with French.

both are OK, but interjecting prepositions in the middle of sentences like this can be jarring.

The core of the task* was to work on the languages each one was learning, but also to doour respective target language by doing something in a group, which is always more fun and motivating than doing it alone.

*I think challenge or activity is a better word. When I think of task, it's a small objective

'by' would describe how you're going to do the activity. 'but also' is something additional.

Today, that the challenge is closending*, it’s time to look back and assess whatere I’ve gainimproved and what areas I need to reinforcework on.

*or you could say..."coming to a close"

The positives are that I could catchhave caught up with my English studies after not having done much in the past 2 years, and to my surprise, I’ve continued to improve in spite of not having worked on my English actively.

Another thing that makes me closeend these 30 days with a smile is knowing that I never took the challenge lightly, quite the contrary, every day I spareent some time to writeing something meaningful to me.

'close' is more often associated with something physically closing, like a store or a door. But we do say things like 'closing ceremony'...English is a weird

In fact, if I had written something quickly just to keep up with the challenge, now I’d be quite dissatisfied with myself now.

I wish I could explain why, but the word order sounds more natural this way.

I admit that for many years, I’ve proudly refused to even consider studying that aspect of grammar.

However, since I’m looking forward to the C1 certificate, I have to make peace with the idea that English, in this regard, is different from Spanish.

Luckily, I have engraved in my mind, the first time I wrote a paragraph in English, it took me the whole afternoon.