I had thought that Japanese learners in overseas would easily read Japanese sentences filled with hiragana rather than ones filled with kanji until I joined the LangCorrect community, but it wasn't true.

It seemed s to me (that) there are three reasons why they hadve the difficulty.

At first, Japanese sentences areit's hard to separate words from ain Japanese sentences, since there isare no spaces in between words.

I don't know why they don't havere are no spaces, but it is useful to pufit as many contentswords as possible into a limited space, like tin Twitter.

Japanese particles are vague and vary depending on the context.

Unfortunately, ithat determines how you (includinggood of a command of Japanese speople) have a good command of Japanese even if you haveakers (including native speakers) have, even if they still posess a lot of vocabulary.

I've altered this sentence quite a bit in an attempt to make it sound more natural, let me know if you need an explanation.

Finally, it's hard to figure out the meaning of words from spelling alone.

Or rathern the other hand, you can easily look words written in kanji up in a dictionary.

I'm going to talk about the same difficulties in English aton my next post.

I had thoughtused to think that Japanese learners in overseas wcould easily read Japanese sentences filled with hiragana (rather than ones filled with kanji until). But when I joined LangCorrect community, but itI learned that this wasn't truhe case.

I broke this up into two sentences because what you had was rather long. Always "overseas," never "in overseas." I put "rather than ones filled with kanji" in parentheses because it is what I would call an "aside," like a note to the reader in case they don't know the difference.

You could also end with "but this wasn't true." I wrote "this wasn't the case" instead because it's just a slight bit more natural, at least to my ear.

It seemed that there are three reasons why they hadthat explain theis difficulty.

Your sentence is correct except for "they had" – should be present tense, "they have." What I wrote is a bit more concise.

At first, Japanese sentences are hard to separate words from a sentence, since there is no space between words.

I don't know why they don't have, but it is usefuluse spaces, but I guess it's convenient to put as many contentharacters as possible into a limited space like tTwitter.

"why they don't have" definitely has to take a direct object. "Convenient" is a more precise adjective than "useful" in this case. And Twitter is capitalized because it's a proper noun.

Second, non-native speakers are hardly able to tell Japanese particles ("joshi") from nouns, verbs, and adjectives.

Unfortunately, it determines how you (including Japanese people) have a good command of Japanese even if you have a lot of vocabulary.

This sentence is a little unclear. Do you mean that the use of Japanese particles shows if a person does or doesn't have a good command of the language? If so, I'd say it like this:

"Even if you have a rich vocabulary, you need to be able to use particles in order to have a good command of the Japanese language. This is the case for native and non-native speakers alike."

I'm going to talk about the same difficulties in English atin my next post.

I had thought that Japanese learners in overseas would easily read Japanese sentences filled with hiragana rather than ones filled with kanji until I joined the LangCorrect community, but it waisn't true.

Past tense when describing something from a point of view set in the past is often fine, but here the present tense makes more sense to me because it's implicitly describing what you believe now.

It seemed that there are three reasons why they had the difficulty.

"Have the difficulty" may be better given that the rest is written in the present tense.

At first, Japanese sentences areit is hard to separate words from ain Japanese sentences since there isare no spaces between words.

I don't know why they don't have spaces, but it is useful to put as many contents as possible into a limited spacespace with limits on how much one can write, like tTwitter.

"Have" needs a clear object.

Or more simply, "character limits," but outside of Twitter and other internet spaces, that may be too specific.

Second, non-native speakers are hardly able to tell Japanese particles, "joshi," from nouns, verbs, and adjectives.

Japanese particles are vague and vary depending on the context.

Or rathern the other hand, you can easily look words written in kanji up in a dictionary.

I'm going to talk about the same difficulties in English at next post.

"Similar difficulties" may be better, because they're not necessarily the same. But this depends on the overall point that you want to make as well.