Bu kez sözünü tutmalısın.

Bu kez means this time or on this occasion.

• bu = this
• kez = time, occasion, instance

In this sentence, it suggests contrast with earlier occasions, often with the feeling of not like before or for once, this time.

Because kez is used for an occurrence/instance, while zaman usually refers to time more generally.

So:

• bu kez = this time, this occasion
• bu zaman is not natural here

A native English speaker might want to translate this time word-for-word with zaman, but Turkish normally uses kez, sefer, or defa in this kind of sentence.

Sözünü tutmak is an idiomatic expression meaning to keep your word or to keep your promise.

Literally:

• söz = word, promise
• tutmak = to hold, keep

So the literal sense is something like to keep/hold your word, which matches the English idiom keep your word quite well.

Because sözünü contains extra grammar:

• söz = word / promise
• -ün = your
• -ü = accusative ending

So:

• sözün = your word / your promise
• sözünü = your word / your promise as the definite direct object

The full structure is:

• söz-ün-ü
• word-your-ACC

Since tutmak is acting on a specific thing, your promise, Turkish marks it as a definite object.

Because Turkish often expresses this idea as one’s word/promise.

Compare:

• söz vermek = to make a promise
• sözünü tutmak = to keep your promise / keep your word

This is very natural in Turkish. English can say both keep your promise and keep your word, and Turkish is doing something similar.

Tutmalısın breaks down like this:

• tut- = keep
• -malı = must / should
• -sın = you (singular)

So:

• tutmalısın = you must keep / you should keep

This is all one word because Turkish adds grammatical endings directly to the verb stem.

It can mean either, depending on context.

The suffix -malı / -meli expresses necessity, obligation, or strong advice. In English, that may come out as:

• should
• ought to
• must

In Bu kez sözünü tutmalısın, the tone is usually fairly strong, so must is often the best translation: This time you must keep your promise.

But in a softer context, it could also feel like you should keep your promise.

Because Turkish usually leaves the subject pronoun out when it is already clear from the verb ending.

Here, -sın in tutmalısın already tells us the subject is you singular.

So:

• Bu kez sözünü tutmalısın. = natural
• Sen bu kez sözünü tutmalısın. = also possible, but more emphatic

Adding sen gives extra emphasis, like You must keep your promise this time.

The sentence follows a very natural Turkish order:

• Bu kez = time expression
• sözünü = object
• tutmalısın = verb

Turkish often places the verb at the end. So this structure is very typical.

English learners sometimes expect a closer match to English word order, but Turkish normally prefers the verb-final pattern.

Yes, Turkish word order is flexible, but changing it changes the emphasis.

For example:

• Bu kez sözünü tutmalısın. = neutral/natural
• Sözünü bu kez tutmalısın. = stronger focus on this time
• Sen bu kez sözünü tutmalısın. = stronger focus on you

So the original sentence is the most straightforward version, but other orders are possible for emphasis.

Yes. All of these can mean time/occasion in this context:

• bu kez
• bu sefer
• bu defa

They are all natural, though the tone may vary slightly by speaker or situation.
Bu kez sounds completely standard and fits this sentence very well.

Because the sentence is expressing obligation, not just a future event.

• tutmalısın = you must/should keep
• tutacaksın = you will keep

So:

• Bu kez sözünü tutmalısın. = This time you must keep your promise.
• Bu kez sözünü tutacaksın. = This time you will keep your promise.

The second one can sound more like a prediction, command, or insistence, while the original clearly expresses necessity.

Not necessarily.

Sözünü just means your word / your promise. The sentence does not specify who the promise was made to. It could be:

• a promise made to the speaker
• a promise made to someone else
• a promise in a general sense

The context would tell you that.

Then the possessive and the verb ending both change:

• Bu kez sözünüzü tutmalısınız.

Breakdown:

• sözünüzü = your promise (plural or formal you)
• tutmalısınız = you must keep (plural or formal)

So the original sentence is for one person, informal singular.