A bBirthday, a "P" and the aAnswer to eEverything (fFirst pPart) A Birthday, a "P" and the Answer to Everything (First Part)

Proper capitalization in your title.

Yesterday I went to a friend of mine's birthday party. Yesterday I went to a friend's birthday party.

Your sentence is grammatically correct, but my correction is simpler and natural

Yesterday I went to a friend of mine'’s birthday party. Yesterday I went to a friend’s birthday party.

“Friend of mine’s” is awkward

I might have mentioned it in one of my texposts here on Langcorrect. I might have mentioned it in one of my posts here on Langcorrect.

There are a few specific things we use "text" to refer to, it's kind of complex to explain but it tends to focus on the aspect of it being a bunch of character strings and not so much the content. ie. "copy and paste the text," "highlight the text," note that in this usage "text" is always singular. We also call typed messages sent through phone service "texts," and we call books "texts" but that sounds kind of academic/archaic It doesn't sound natural here.

I might have mentioned it in one of my texts here on LangcCorrect. I might have mentioned it in one of my texts here on LangCorrect.

Capitalize “LangCorrect” consistently.

Even though the room where the language exchange takes place was closed, my friend also invited some people that work at the organisation that manages thise event, so we couldand they allowed us to party there. Even though the room where the language exchange takes place was closed, my friend also invited some people that work at the organisation that manages the event, and they allowed us to party there.

This sentence is kind of confusing without clarifying that they gave you access to the room

Even though the room where the language exchange takes place was closed, my friend also invited some people thatwho work at the organiszation that manages this event, so we could party there. Even though the room where the language exchange takes place was closed, my friend also invited some people who work at the organization that manages this event, so we could party there.

Use “who” instead of “that” for people.

everyone had to get dressed as something that starts with the same first letter asof their name. everyone had to get dressed as something that starts with the first letter of their name.

everyone had to get dressed as something that startsed with the same first letter as their name. everyone had to get dressed as something that started with the same first letter as their name.

Past tense “started” matches the invitation context.

Since my name is Pietro, my letter was "P"." Since my name is Pietro, my letter was "P."

commas and periods go inside quotes

Since my name is Pietro, my letter was "P"." Since my name is Pietro, my letter was "P."

Place the period inside the quotation marks.

Initially I thought of some unoriginal costumes (like "thea pirate"), but then I had an idea: Initially I thought of some unoriginal costumes (like "a pirate"), but then I had an idea:

Initially I thought of some unoriginal costumes (like "thea pirate"), but then I had an idea: Initially I thought of some unoriginal costumes (like a pirate), but then I had an idea:

Initially, I thought of some unoriginal costumes (like "the pirate"), but then I had an idea: Initially, I thought of some unoriginal costumes (like "the pirate"), but then I had an idea:

Add a comma after “Initially.”

I'm going to would dress up as a "problem"! I would dress up as a "problem"!

“I would” to match past tense narration.

I wrote a funny and meaningless math formula and I generated with ChatGPTused ChatGPT to generate a math frame. I wrote a funny and meaningless math formula and I used ChatGPT to generate a math frame.

Then I did the same thing withon the back of the image, where I wrote the solution: 42, a reference to the book “The Hitchhiker's Guide to the Galaxy”.” Then I did the same thing on the back of the image, where I wrote the solution: 42, a reference to the book “The Hitchhiker's Guide to the Galaxy.”

Use “on the back” instead of “with the back.”

I even bought some post-its whereon which I wrote some funny sentences to "help" people solve the problem. I even bought some post-its on which I wrote some funny sentences to "help" people solve the problem.

sounds like you bought post-its at the location where you wrote funny sentences

I wrote sentences like "It seems easy, but it's not" or "We explained this in class"." I wrote sentences like "It seems easy, but it's not" or "We explained this in class."