Onun bunu zaten bildiğini varsayma.

A helpful breakdown is:

• onun = his / her / its in the genitive form, or more naturally here, that he/she
• bunu = this / it as a direct object
• zaten = already
• bildiğini = that he/she knows
• varsayma = do not assume

So the structure is roughly:

[Onun bunu zaten bildiğini] varsayma
= Do not assume [that he/she already knows this].

The part onun bunu zaten bildiğini is a subordinate clause functioning as the object of varsayma.

Because this is not a simple main clause like O bunu biliyor.

In Turkish, when a clause is turned into a noun-like clause with endings such as -DIK, its subject is usually put in the genitive case. That is why you get:

• o = he / she / it
• onun = his / her / its, and in this kind of structure, that he/she

So:

• O bunu biliyor = He/she knows this
• Onun bunu bildiği = that he/she knows this

This genitive + nominalized verb pattern is very common in Turkish.

bildiğini is a compact grammatical form. It can be broken down like this:

• bil- = know
• -dik / -dık / -diği type ending = a nominalizing ending that helps form that someone knows
• -i = third person possessive agreement
• -ni = accusative, with a buffer n

A simplified step-by-step view:

• bil-
• bildik
• bildiği = his/her knowing, or more naturally that he/she knows
• bildiğini = that whole clause in the accusative form

So bildiğini does not just mean knowing. Here it means that he/she knows.

Also note the sound change:

• underlying k often becomes ğ before a vowel
• so bildik + i becomes bildiği

Because bunu is the direct object form of bu.

The verb bilmek normally takes a direct object when you say what someone knows. Here, the thing known is this, so Turkish uses the accusative object form:

• bu = this
• bunu = this, as a specific direct object

So:

• bu by itself = this
• bunu bilmek = to know this

That is why bunu is natural here.

Because the whole subordinate clause is the object of varsayma.

The sentence is effectively:

• Do not assume [that he/she already knows this].

In Turkish, that bracketed clause is expressed as a noun-like structure, and because it is the direct object of varsaymak, it appears in the accusative:

• onun bunu zaten bildiği = that he/she already knows this
• onun bunu zaten bildiğini = that same clause as the object of assume

So there are two object relationships here:

1. bunu is the object of bilmek
2. bildiğini is the object of varsaymak

Here varsayma is a negative imperative addressed to one person.

It comes from:

• varsaymak = to assume
• stem: varsay-
• negative imperative ending: -ma

So:

• varsay = assume
• varsayma = do not assume

This is the singular, informal form, used for speaking to one person.

Related forms:

• varsaymayın = do not assume, said politely or to more than one person
• varsayma! = same thing, with more explicit imperative punctuation

In another context, -ma can also make verbal nouns, but here the sentence structure clearly shows it is an imperative.

Turkish normally puts the main verb at the end, and that is exactly what happens here.

Structure:

• onun = subject of the embedded clause
• bunu = object of the embedded clause
• zaten = adverb
• bildiğini = embedded predicate in nominalized form
• varsayma = main verb

So the sentence ends with the main action: do not assume.

A very literal order would look like:

[his/her this already knowing-that] do-not-assume

That sounds strange in English, but it is normal Turkish structure.

Yes, often it can be omitted if the context already makes the person clear.

You could say:

Bunu zaten bildiğini varsayma.

That still means Do not assume that he/she already knows this, with the third-person subject understood from the verb form.

However, keeping onun can be useful when:

• you want to be clear about who you mean
• there is more than one possible person in the conversation
• you want a bit more emphasis

So both are possible, but onun adds clarity.

It can mean either one.

Turkish third-person singular pronouns do not mark gender:

• o = he / she / it
• onun = his / her / its, or in this structure, that he/she/it

So the exact gender comes only from context, not from the word itself.

Here zaten most naturally means already.

So it shows that the speaker is talking about prior knowledge: do not assume the person knows it beforehand.

But zaten is a flexible word and can also mean things like:

• anyway
• as it is
• after all

For example, in other contexts it can sound more like besides or in any case. In this sentence, though, already is the best match.

Yes. This is a very normal and natural pattern in Turkish.

Turkish often expresses that-clauses by turning them into noun-like clauses. So instead of a separate word like English that, Turkish often uses a nominalized form such as:

• onun geldiğini biliyorum = I know that he/she came
• onun haklı olduğunu düşünüyorum = I think that he/she is right
• onun bunu zaten bildiğini varsayma = Do not assume that he/she already knows this

So if this structure feels dense at first, that is normal. It is one of the key patterns in Turkish grammar.