[Grammar] In English or At English?

[Pakhavit]

Dear teacher, can you tell me which one is correct or better?

I'm really bad in English.
I'm really bad at English.

I'm really good at swimming.
I'm really good in swimming.

I'm really bad (in/at) English, so I join here for practice.
I'm really bad (in/at) English. So I join here for practice.

I remember that period+conjunction is acceptable now, but would a comma as a linker for two sentences is better? I feel like period+conjunction looks better in a long sentence with many clauses and prepositions.

Thank you very much.

 

[emsr2d2]

Dear teacher, can you tell me which one is correct or better?

I'm really bad in English.
I'm really bad at English.

I'm really good at swimming.
I'm really good in swimming.

I'm really bad (in/at) English, so I join here for practice.
I'm really bad (in/at) English. So I join here for practice.

I remember that period+conjunction is acceptable now, but would a comma as a linker for two sentences is better? I feel like period+conjunction looks better in a long sentence with many clauses and prepositions.

Thank you very much.

It's "I'm very good at swimming/English etc"

Period + conjunction might be acceptable now, but it still looks very strange to me. I prefer the comma in your example.