Yesterday, I didn't slept because I felt sick. Yesterday, I didn't slept because I felt sick.

Or: I didn’t get much sleep yesterday because I was sick. Or: I didn’t sleep at all yesterday because I was sick. ^ more natural

Yesterday, I didn't sleept because I felt sick. Yesterday, I didn't sleep because I felt sick.

"didn't" is already past tense. There's a list of verbs of being and "do, does, did" are among them, When active verbs follow them, they usually take the infinitive form. I'm sorry to hear that you're not feeling very good!

Yesterday, I didn't sleept because I felt sick. Yesterday, I didn't sleep because I felt sick.

You don't need to conjugate "sleep" in this case.

This could be badClearly this situation isn’t ideal, but I'’d rather think of ithis as an opportunity to write more journals and to helping other's inwith their Spanish. on this website. Clearly this situation isn’t ideal, but I’d rather think of it as an opportunity to write more journals and to help others with their Spanish on this website.

Very natural.

This could be bad but I'd rather think of this as an opportunity to write more journals and to helping other's inwith their Spanish. This could be bad but I'd rather think of this as an opportunity to write more journals and to help others with their Spanish.

I edited the sentence for parallel construction. "Others" in this sentence is a plural noun instead of a possessive so that's why I deleted the apostrophe.

This could beSome may think this is bad, but I'd rather think of this as an opportunity to write more journals and helping other's inwith their Spanish. Some may think this is bad, but I'd rather think of this as an opportunity to write more journals and helping others with their Spanish.

Comma before "but"