Я часто думаю о нём вечером.

I often think about him in the evening.

Breakdown of Я часто думаю о нём вечером.

• я: I
• часто: often
• вечером: in the evening
• о: about

Questions & Answers about Я часто думаю о нём вечером.

Why is it о нём after думаю? Why not его?

Because думать in the meaning to think about normally takes о + Prepositional case:

думать о ком? о чём? = to think about whom? about what?

So:

• о нём = about him
• о ней = about her
• о тебе = about you

Using его here would be wrong, because его is not the form required after о in this meaning.

What case is нём, and why does it begin with н-?

нём is the Prepositional singular form of он.

The full pattern is:

• он
• о нём

Russian third-person pronouns often add an initial н- after prepositions:

• у него
• к ней
• с ним
• о нём

So the н- is not random; it is a regular feature of these pronoun forms after prepositions.

Why is it written нём with ё? Will I sometimes see нем?