Onu yedinci gün gördüm.

A word-by-word breakdown is:

• onu = him / her / it as the direct object
• yedinci = seventh
• gün = day
• gördüm = I saw

So the structure is roughly:

• onu → object
• yedinci gün → time expression
• gördüm → verb

Very literally, the order is something like him/her/it seventh day saw-I.

Because onu is the accusative form of o.

• o = he / she / it / that in the basic form
• onu = him / her / it / that one as a direct object

Since görmek means to see, and the person/thing seen is the direct object, Turkish uses onu here.

Compare:

• O geldi. = He/She came.
• Onu gördüm. = I saw him/her/it.

Also, Turkish normally marks specific/definite direct objects with the accusative, and pronouns like o are inherently specific, so onu is required.

It can mean any of those, depending on context.

Turkish does not normally distinguish gender in third-person singular pronouns the way English does. So:

• o can mean he, she, or it
• onu can mean him, her, or it

In some contexts, it can also mean that one.

So you need context to know which English pronoun is best.

Yedinci means seventh. It is the ordinal form of yedi (seven).

Turkish makes ordinals with a suffix that appears in different vowel-harmony forms, such as:

• birinci = first
• ikinci = second
• üçüncü = third
• yedinci = seventh

So:

• yedi = seven
• yedinci = seventh

When it comes before a noun, it works like an adjective:

• yedinci gün = the seventh day

Because Turkish often uses bare time expressions without a preposition or case ending where English uses on, in, or at.

So yedinci gün can naturally mean:

• on the seventh day

That is very normal Turkish.

Yedinci günde is possible in some contexts, but it is not the most neutral way to say this here. The bare form yedinci gün is a common and natural time expression.

This is something English speakers often have to get used to: English needs a preposition, but Turkish often does not.

Because Turkish has no definite article equivalent to English the.

Turkish often leaves definiteness to be understood from:

• context
• word order
• case marking
• the type of expression used

In English, seventh day almost always becomes the seventh day in natural translation. In Turkish, yedinci gün already carries that meaning without any separate word for the.

Because the verb already shows the subject.

Gördüm includes the ending for I, so Turkish does not need ben unless you want emphasis or contrast.

• gördüm = I saw
• ben gördüm = I saw with extra emphasis, like I was the one who saw

So:

• Onu yedinci gün gördüm. = normal
• Ben onu yedinci gün gördüm. = possible, but more emphatic

This omission of subject pronouns is very common in Turkish.

Gördüm comes from the verb görmek (to see).

The parts are:

• gör- = verb stem
• -dü- = past tense
• -m = first person singular (I)

So:

• gör-dü-m = I saw

The infinitive görmek ends in -mek, but that -mek is just the dictionary form ending. When the verb is conjugated, you use the stem gör-.

Because of vowel harmony.

The Turkish past tense suffix has several forms:

• -dı
• -di
• -du
• -dü

Which one appears depends on the last vowel of the verb stem.

In gör-, the vowel is ö, which is a front rounded vowel, so the matching past suffix is -dü:

• gör + dü + m → gördüm

So it is not arbitrary; it follows a regular sound pattern.

Turkish word order is flexible, although the verb often stays at the end.

So these are all possible, with different emphasis:

• Onu yedinci gün gördüm.
• Yedinci gün onu gördüm.
• Onu gördüm yedinci gün. — possible in some contexts, but less neutral

A useful rule is that the element right before the verb often gets stronger focus.

So in:

• Onu yedinci gün gördüm

the phrase yedinci gün is in the position that often gets emphasis. It can sound a bit like:

• I saw him/her on the seventh day

with special attention on when I saw him/her.

So yes, the order can change, but the emphasis changes too.