Sözlük kullanmak, karmaşık metinleri anlamak için kesinlikle faydalı bir yöntemdir.

Because kullanmak is the infinitive, meaning to use / using.

In this sentence, Sözlük kullanmak means using a dictionary or more literally to use a dictionary. Turkish often uses the infinitive this way when an action itself is the subject of the sentence.

So:

• sözlük = dictionary
• kullanmak = to use
• sözlük kullanmak = using a dictionary

Here, that whole phrase is the subject:

• Sözlük kullanmak ... faydalı bir yöntemdir.
• Using a dictionary ... is a useful method.

Turkish does not use articles the same way English does.

So sözlük can mean:

• a dictionary
• the dictionary
• just dictionary in a general sense

In this sentence, sözlük kullanmak is understood generically as using a dictionary.

Turkish often leaves this kind of noun unmarked when speaking generally. You do not need bir here, because the focus is on the action using a dictionary, not on one dictionary as a specific object.

Compare:

• Sözlük kullanmak = using dictionaries / using a dictionary in general
• Bir sözlük kullanmak = using one dictionary / using a particular dictionary-like item

The first is more natural here.

Because it is the direct object of anlamak and it refers to a specific class of things: the complex texts or complex texts as the target to be understood.

Breakdown:

• metin = text
• metinler = texts
• metinleri = the texts / texts as a definite direct object

In Turkish, direct objects often take the accusative ending when they are definite or specific.

So:

• karmaşık metinler anlamak would sound like to understand complex texts in a more indefinite or less natural way here
• karmaşık metinleri anlamak = to understand the complex texts / complex texts (as a specific target)

In practice, after a phrase like ... için meaning in order to ..., Turkish commonly uses the accusative when the object is understood as definite or clearly identified.

Also note that metinleri can sometimes be ambiguous in isolation, because it can also mean their texts, but in this sentence it clearly means the texts as the object.

İçin means for or in order to.

So:

• anlamak = to understand
• anlamak için = in order to understand / for understanding

The phrase karmaşık metinleri anlamak için means:

• in order to understand complex texts
• for understanding complex texts

This structure is extremely common in Turkish:

• öğrenmek için = in order to learn
• görmek için = in order to see
• başarmak için = in order to succeed

So the sentence is saying that using a dictionary is a useful method for the purpose of understanding complex texts.

Kesinlikle means definitely / absolutely / certainly.

It is placed before faydalı bir yöntemdir to strengthen the statement:

• kesinlikle faydalı bir yöntemdir = is definitely a useful method

Turkish word order is flexible, but this position is very natural because kesinlikle modifies the whole predicate.

Possible placements include:

• Sözlük kullanmak ... kesinlikle faydalı bir yöntemdir.
• Sözlük kullanmak kesinlikle ... faydalı bir yöntemdir.

The version in your sentence sounds natural and clear.

Because the sentence is not just saying it is useful. It is saying it is a useful method.

Compare:

• faydalıdır = it is useful
• faydalı bir yöntemdir = it is a useful method

So:

• Sözlük kullanmak ... faydalıdır.
  = Using a dictionary ... is useful.

• Sözlük kullanmak ... faydalı bir yöntemdir.
  = Using a dictionary ... is a useful method.

The second version is a little more specific and formal because it classifies dictionary use as a method.

-dir is a form of the Turkish copula, roughly corresponding to is in English, but with a slightly formal, emphatic, or general-truth tone.

So:

• yöntem = method
• bir yöntem = a method
• bir yöntemdir = is a method

In everyday spoken Turkish, this ending is often omitted:

• Sözlük kullanmak ... faydalı bir yöntem.

That still means the same thing, and it is very natural in speech.

With -dir, the sentence sounds:

• more formal
• more written
• more assertive
• more like a general statement or fact

So yöntemdir is not wrong or unusual at all; it just sounds more formal than casual speech.

Because bir here means a.

• faydalı bir yöntem = a useful method

This is one place where Turkish does use bir similarly to English a/an.

Without bir, the phrase would sound less natural here:

• faydalı yöntemdir is possible in some contexts, but it is less standard in this sentence
• faydalı bir yöntemdir is the normal way to say is a useful method

So bir is needed because the noun phrase is being presented as a useful method.

Not always strictly necessary, but it is understandable and acceptable.

The comma separates the subject phrase from the rest of the sentence:

• Sözlük kullanmak, karmaşık metinleri anlamak için kesinlikle faydalı bir yöntemdir.

Since Sözlük kullanmak is a fairly long verbal subject and the sentence continues with a purpose phrase, the comma helps readability.

You may also see the sentence written without a comma:

• Sözlük kullanmak karmaşık metinleri anlamak için kesinlikle faydalı bir yöntemdir.

That is also possible.

So the comma is mainly a stylistic or clarity choice, especially in writing.

The word order is flexible, but the original sentence is natural and standard.

The basic idea is:

• Sözlük kullanmak = subject
• karmaşık metinleri anlamak için = purpose phrase
• kesinlikle faydalı bir yöntemdir = predicate

Turkish often puts important descriptive material before the final verb or copular ending. That is why the sentence ends with yöntemdir.

Possible variations include:

• Karmaşık metinleri anlamak için sözlük kullanmak kesinlikle faydalı bir yöntemdir.
• Sözlük kullanmak kesinlikle karmaşık metinleri anlamak için faydalı bir yöntemdir.

These are still understandable, but the original version is smooth and well balanced.

So the sentence is not rigidly fixed, but not every rearrangement sounds equally natural.

Yes. Both are reasonable depending on the style of translation.

Literally:

• anlamak için = in order to understand

But in smoother English, you might also say:

• for understanding complex texts

So this phrase can be interpreted in a few close ways:

• in order to understand complex texts
• to understand complex texts
• for understanding complex texts

The Turkish structure itself clearly expresses purpose.

A useful way to see it is:

[Infinitive phrase] + [purpose phrase] + [predicate]

So:

• Sözlük kullanmak = using a dictionary
• karmaşık metinleri anlamak için = in order to understand complex texts
• kesinlikle faydalı bir yöntemdir = is definitely a useful method

This is a very common Turkish pattern:

• [Doing X], [for doing Y], is useful/helpful/important.

For example:

• Not almak, bilgileri hatırlamak için faydalı bir yöntemdir.
  = Taking notes is a useful method for remembering information.

Once you recognize that pattern, this sentence becomes much easier to understand structurally.