Akşam köftenin yanına patates püresi koyacağım.

Yes. Akşam can work as a time expression on its own, and Turkish often puts time expressions near the beginning of the sentence.

So Akşam köftenin yanına patates püresi koyacağım is perfectly natural.

A few related options:

• Akşam = in the evening / tonight depending on context
• Bu akşam = this evening / tonight
• Akşamleyin = in the evening (a bit more explicitly time-related)

Turkish does not require a preposition like in here. The time word can simply appear by itself.

Because of the expression X-in yanına.

Here, yan means side, and yanına means to its side / next to it / alongside it. To say to the side of the meatballs, Turkish uses the genitive form on köfte:

• köfte = meatball
• köftenin = of the meatball / the meatball’s

So:

• köftenin yanına = to the side of the meatball(s)

This is a very common structure in Turkish:

• tavuğun yanına pilav = rice with chicken / rice on the side of the chicken
• balığın yanına salata = salad with fish / salad on the side of the fish

It is köfte + -nin.

The suffix -(n)in / -(n)ın / -(n)un / -(n)ün is the genitive ending, chosen by vowel harmony.

Since köfte ends in a vowel, the form uses the buffer consonant n:

• köfte
• köfte-nin
• köftenin

So this is not irregular; it is just normal suffixation.

Yanına comes from:

• yan = side
• -ı = possessive marker in this construction
• -na = dative to

So yanına literally means something like to its side.

In natural English, in food contexts, this often means:

• with
• on the side of
• alongside

That is why köftenin yanına patates püresi koyacağım means serving mashed potatoes with the meatballs, not physically placing them next to meatballs in a random spatial sense.

This is a very common learner question.

• yanında = at its side / beside it
• yanına = to its side / to go next to it

So:

• köftenin yanında = beside the meatballs / with the meatballs
• köftenin yanına = to the side of the meatballs

In this sentence, koymak often goes naturally with yanına, because koymak implies placing something somewhere:

• köftenin yanına koyacağım = I’ll put it next to / alongside the meatballs

This is a Turkish noun compound.

• patates = potato
• püre = purée / mash
• patates püresi = potato purée / mashed potatoes

In Turkish, when one noun modifies another in this kind of compound, the second noun usually takes a third-person possessive ending:

• elma suyu = apple juice
• domates çorbası = tomato soup
• patates püresi = potato purée

So püresi is not meaning his/her purée here. It is just the normal compound form.

Great question. In Turkish, the accusative is usually used for a specific, definite direct object.

Compare:

• Patates püresi koyacağım. = I’ll put/serve some mashed potatoes.
• Patates püresini koyacağım. = I’ll put/serve the mashed potatoes.

In your sentence, patates püresi is non-accusative, which can sound more like an indefinite food item being served as part of the meal.

If you said patates püresini, it would sound more definite, as if a particular mashed potato dish has already been identified.

Koyacağım comes from the verb koymak = to put / place / add / serve depending on context.

Formation:

• koy- = verb stem
• -acak / -ecek = future tense
• -ım / -im / -um / -üm = I

Because of sound changes, it becomes:

• koy + acak + ım
• koyacağım

So koyacağım = I will put / I’m going to put

In a food context, koymak can often be understood as:

• put on the plate
• serve
• add as a side

Because Turkish usually does not need the subject pronoun when the verb ending already shows the person.

Here, -acağım tells you the subject is I.

So:

• koyacağım = I will put

You could say Ben akşam köftenin yanına patates püresi koyacağım, but ben is usually omitted unless you want emphasis or contrast.

For example:

• Ben köftenin yanına patates püresi koyacağım, o ise pilav yapacak. = I’ll serve mashed potatoes with the meatballs, but he/she will make rice.

Turkish word order is flexible, though some orders sound more neutral than others.

Your sentence is natural:

• Akşam köftenin yanına patates püresi koyacağım.

But you could also hear:

• Köftenin yanına akşam patates püresi koyacağım.
• Patates püresi akşam köftenin yanına koyacağım.
• Akşam patates püresini köftenin yanına koyacağım.

The most neutral pattern in Turkish often puts the verb at the end, while earlier parts can move around for emphasis, topic, or rhythm.

So the verb staying at the end is the most important part; the rest is somewhat flexible.

Yes, other verbs are possible depending on nuance.

Koymak is very common and practical. In food contexts it can mean:

• put on the plate
• add
• serve

Possible alternatives:

• Hazırlayacağım = I will prepare
• Yapacağım = I will make
• Ek­leyeceğim = I will add
• Servis edeceğim / sunacağım = I will serve

But koyacağım sounds very natural in everyday Turkish when talking about what you’ll put next to a main dish.

In English, meatballs would often be plural, but Turkish can use the singular noun in a generic food sense.

So köftenin yanına can naturally refer to with the meatballs as part of a meal.

Turkish often leaves nouns in singular form when the plural is not especially important or is understood from context, especially with food and menu-style expressions.

If needed, you could say:

• köftelerin yanına = next to the meatballs

But köftenin yanına can still sound very natural when talking about the dish as a whole.