Kahvenin yanında biraz su istiyorum.

Because yanında is part of a structure that usually takes a noun in the genitive form.

• kahve = coffee
• kahvenin = of the coffee

In Turkish, X-in yanında literally means at the side of X.

So:

• kahvenin yanında = at the coffee’s side / with the coffee

This is a very common pattern:

• evin yanında = next to the house
• masanın yanında = next to the table

So kahvenin is there because yanında depends on it.

Here, yanında means something like beside it, next to it, or more naturally in this context, with it.

It comes from:

• yan = side
• -ı = its
• -nda = in/on/at

So yanında literally means at its side.

In a café or restaurant context, kahvenin yanında is a very natural way to say:

• with the coffee
• on the side with the coffee
• served alongside the coffee

So even though the literal meaning is about physical position, the real meaning here is more like served with.

Because Turkish sentence structure works differently from English.

A very common neutral Turkish word order is:

• context / place / time
• object
• verb

So in this sentence:

• Kahvenin yanında = with the coffee
• biraz su = some water
• istiyorum = I want

The verb normally comes at the end in Turkish. That is one of the biggest differences from English.

Biraz means:

• a little
• some
• a bit of

So biraz su means:

• a little water
• some water

It makes the request sound natural. If you just said su istiyorum, that means I want water, which is also grammatical, but biraz su often sounds softer or more natural when asking for a small amount.

Because su is an uncountable noun, like water in English.

In English, you usually say:

• some water not
• waters

Turkish works similarly here:

• biraz su = some water

You would only pluralize su in special cases, not in this everyday meaning.

Good question. In Turkish, direct objects are not always marked the same way.

Here, biraz su is an indefinite object, so it does not take the accusative ending.

• biraz su istiyorum = I want some water

If the object were specific or definite, you would usually use the accusative:

• suyu istiyorum = I want the water

So the lack of an ending on su helps show that this means some water, not a specific previously known water.

İstiyorum means I want.

It can be broken down like this:

• iste- = want
• -iyor = present continuous / present tense marker
• -um = I

So:

• istiyorum = I am wanting / I want

In modern Turkish, this form is the normal way to say I want.

Yes, it is natural and understandable. It sounds direct but normal.

However, in a café or restaurant, Turkish often prefers slightly softer requests. For example:

• Kahvenin yanında biraz su alabilir miyim? = Could I have some water with the coffee?
• Kahvenin yanında biraz su rica edebilir miyim? = May I request some water with the coffee?

Your original sentence is still fine, especially in casual situations:

• Kahvenin yanında biraz su istiyorum.

It is simply a bit more direct, like I want some water with the coffee.

Sometimes, but it changes the feel.

• kahve ile or kahveyle = with coffee
• kahvenin yanında = beside the coffee / served with the coffee

In this context, kahvenin yanında is more idiomatic if you mean something served on the side with the coffee.

For example:

• Kahveyle süt istiyorum = I want milk with the coffee
• Kahvenin yanında su istiyorum = I want water served alongside the coffee

So ile means with, but yanında is especially natural for on the side / alongside.

Yes, it can. Literally, it does mean next to the coffee or at the side of the coffee.

Turkish often uses expressions with physical meanings in broader ways. So depending on context:

• literal: next to the coffee
• practical restaurant meaning: with the coffee / alongside the coffee

In this sentence, a native speaker would usually understand the restaurant-style meaning unless the situation strongly suggests physical placement.

That changes the meaning.

• kahvenin yanında = at the side of the coffee / with the coffee
• kahvenin yanına = to the side of the coffee / next to the coffee

The ending -a / -e often shows direction to something.

So:

• kahvenin yanında biraz su istiyorum = I want some water with the coffee
• kahvenin yanına biraz su koyun = Put some water next to the coffee

For your sentence, yanında is the better choice.

A very literal breakdown is:

• kahvenin = of the coffee
• yanında = at its side
• biraz = a little / some
• su = water
• istiyorum = I want

So the full literal sense is:

• At the coffee’s side, I want some water

But natural English is:

• I want some water with the coffee.