l know, Iangcorrectors!

Serifs isare these smaII Iines attached to the end of Ietters to make them ugly.

So, most modern websites started to use San-Serifs(the, fonts without the Serifs), to make their websites cool, just aslike this websitone.

I replaced "websites" to avoid repetition.

However E, everything come with it'ss at a prize, and after this shift, we got this (I,l)!

and fFrom this day, we start to struggle on, it became hard for us to differentiate between the capitalized "i" from the non-capitalized "L".

And from this, we learned a lesson that: we should respect serifs!

ILangcorrect

l know I, Langcorrectors!

"Langcorrectors" would be a proper noun because it's a name for a group of people
When you say "I know langcorrectors" without the comma, it can mean that you know who Langcorrectors are. Another example is:
"I know Alex, she's in my class."
This is not what you were trying to say.

Serifs isare these smaII Iines attached to the end of Ietters to make them ugly.

"Serifs" is plural, so you need to say "are" instead of "is"

So, most modern websites started to use San-Serifs(, the fonts without the Serifs), to make their websites cool, just aslike this website.

"So" is a transition word/phrase in this sentence, so it needs a comma afterwards.
Instead of saying what you said in the parenthesis, you can just put the phrase in between commas. You can do that with anything you describe. Here's another example:
Alex, a girl from my class, is coming over tonight to study for the test with me.

However E, everything comes with it'sa prize, and after this shift, we got this (I,l)there's the problem differentiating I and l!

However is a transition word/phrase in this sentence, so it needs a comma afterwards.
Everything does not need to be capitalized.
I changed the end of the sentence to make it sound better.

and fFrom this day, we start to struggle to differentiate betweescern the capitalized "i" from the non-capitalized "L".

"discern" has a similar definition to "differentiate", but it sounds better here.

And from this, we learned a lesson that we should respect serifs!

Serifs isare these smaII Iines attached to the ends of Ietters to make them ugly.

Bwahh hah ha!

So most modern websites started to use Ssan-Sserifs (the fonts without the Sserifs) to make their websites cool, just aslike this website.

Serif doesn't need to be capitalized. Bonus information: 'san-serif,' 'sans-serif,' and 'sans serif' all appear to be correct.

However E, everything come with it's prize, and after this shift, we got this (I,l)!

If you share this post elsewhere, it would be nice if you could replace '(I,l)' with a larger graphic of those two letters.

and from this day, we start toNow we struggle to differentiate between the capitalized "i" fromand the non-capitalized "L".

'From this day' sounds a little odd because there is no specific day being referred to.

You differentiate A from B, or you differentiate between A and B. You don't need both 'between' and 'from.'