Sentences 2 & 4 are fine. A better way of saying the first sentence would be "I'm afraid of getting married because I've seen many families fall apart". And I'd suggest something like this for the third sentence: "Yesterday I went to bed early to get a good night's sleep"

1 I am afraid of marrying because I see many families falling apart.

1 I am afraid of marrying because I saw many families falling apart. - I am afraid of marriage because I saw plenty of families fall apart. 2 I've spent 8 years learning English. Yes it is correct. 3 Yesterday I went to bed early to sleep myself out. - Yesterday, I went to bed early. 4 Nobody knows whether we are alone in the universe or not. - Nobody knows whether we alone in the universe.

Thank you Talal

I am afraid of marriage.... Yesterday I went to bed early.