Onun sessiz kaldığını ben de varsaydım.

I also assumed that the person had stayed silent.

Breakdown of Onun sessiz kaldığını ben de varsaydım.

• ben: I
• kalmak: to stay
• de: as well
• onun: his

Questions & Answers about Onun sessiz kaldığını ben de varsaydım.

Can you break the sentence down word by word?

Yes:

• onun = his / her / its
• sessiz = silent / quiet
• kaldığını = that he/she remained / stayed
• ben de = I too / I also
• varsaydım = I assumed

A very important point is that onun sessiz kaldığını is not just three separate words in meaning. It works as one clause-like noun phrase: that he/she remained silent. That whole chunk is the object of varsaydım.

Does onun mean his or her?

It can mean either, depending on context.

Turkish third-person singular does not mark gender:

• o = he / she / it
• onun = his / her / its

So from onun alone, you cannot know whether the person is male or female. Context tells you.

Why is it onun, not o?

Because in this kind of embedded clause, Turkish usually puts the subject in the genitive case.

Compare:

• O sessiz kaldı. = He/She remained silent.
• Onun sessiz kaldığını... = that he/she remained silent...

This is a very common Turkish pattern:

• subject of the embedded clause → genitive
• verb of the embedded clause → a nominalized form

So onun is not random here; it is required by the structure.