Kayısı ile mandalinayı aynı tabağa koydum; ikisi de bugün çok tazeydi.

Because in a phrase joined by ile, the case marking is often shown only once, on the last noun of the whole phrase.

So kayısı ile mandalinayı means something like the apricot and the tangerine as one combined object of koydum.

Breakdown:

• mandalina = tangerine
• mandalinayı = the tangerine, marked as a definite direct object
• kayısı ile mandalinayı = the apricot and the tangerine

In Turkish, this is normal. The accusative ending on the last item can apply to the whole coordinated phrase.

Here ile functions like and in English.

Literally, ile often means with, but when it connects two nouns, it can also mean and:

• Ali ile Ayşe = Ali and Ayşe
• çay ile kahve = tea and coffee

So in this sentence, kayısı ile mandalinayı is best understood as the apricot and the tangerine, not as the apricot with the tangerine in a strong instrumental sense.

Because koymak usually takes a destination marked with the dative case -e / -a when you are putting something somewhere.

• tabak = plate
• tabağa = onto / into the plate, to the plate

So:

• aynı tabağa koydum = I put it onto the same plate

Compare:

• tabakta = on the plate, in the plate, describing location
• tabağa = to the plate, describing movement toward it

Since the action is movement to a place, the dative is used.

Aynı means same.

So:

• aynı tabak = the same plate
• aynı tabağa = onto the same plate

It does not take case itself here; the noun after it gets the case ending:

• aynı tabak
• aynı tabağa
• aynı tabakta

This is very common in Turkish adjectives: the adjective stays unchanged, and the noun carries the grammatical marking.

Koydum comes from the verb koymak, meaning to put or to place.

Breakdown:

• koy- = verb stem
• -du- = past tense
• -m = I

So koydum means I put.

The -du changes form according to vowel harmony and consonant voicing, so you may also see forms like -dı, -di, -dü, -tı, -ti, -tu, -tü with other verbs.

İkisi de means both of them.

Breakdown:

• iki = two
• ikisi = the two of them
• ikisi de = both of them

The form ikisi is used when referring back to two specific things already mentioned. Here, those two things are kayısı and mandalina.

So:

• ikisi de bugün çok tazeydi = both of them were very fresh today

Just iki de would not work the same way here.

Here de adds the meaning of too / also / both in a reinforcing way.

With expressions like ikisi de, üçü de, hepimiz de, it helps mean all of them / both of them.

Examples:

• ikisi de geldi = both came
• üçü de hazır = all three are ready

So ikisi de is a set phrase meaning both of them.

This de is a separate word, not the locative ending -de.

Because -ydi adds past meaning, similar to was/were in English.

Breakdown:

• taze = fresh
• tazeydi = was fresh / were fresh

Since the subject is ikisi de = both of them, the English translation uses were, but Turkish still uses the same predicate form tazeydi.

Turkish does not change adjectives for singular vs. plural the way English changes was to were.

The y is a buffer consonant.

When a suffix beginning with a vowel is added after a word ending in a vowel, Turkish often inserts y to make pronunciation easier.

So:

• taze + idi
• becomes taze y di
• written as tazeydi

This happens in many similar forms:

• iyi + idi → iyiydi
• hasta + idi → hastaydı

Bugün means today, and it is placed before the predicate to set the time frame for the statement.

• ikisi de bugün çok tazeydi = both of them were very fresh today

Turkish word order is flexible, but this placement is very natural. It keeps the sentence easy to follow:

1. subject-like expression: ikisi de
2. time: bugün
3. degree + adjective + past copula: çok tazeydi

You could move bugün for emphasis, but the given order is neutral and common.

It is mainly a punctuation choice linking two closely related statements:

• Kayısı ile mandalinayı aynı tabağa koydum
• ikisi de bugün çok tazeydi

The second clause explains or comments on the first, so the semicolon works well. A period would also be possible, and in informal writing you might even see a comma, though the semicolon is clearer.

It does not change the grammar of the Turkish sentence; it just shows a close connection between the two ideas.