O öğrenci çok tembel.

In Turkish, the verb to be is often not expressed in simple present-tense sentences like this one.

So:

• O öğrenci çok tembel. = That student is very lazy.

This is completely normal Turkish.
In the present tense, especially in everyday language, Turkish often uses zero copula—in other words, no separate word for is/am/are.

You may also sometimes see:

• O öğrenci çok tembeldir.

The -dir ending adds a more formal, general, or emphatic tone. In normal conversation, it is usually omitted.

Here, o means that.

A very important point:

• o before a noun usually means that
• o by itself can mean he, she, or it

So:

• o öğrenci = that student
• o çok tembel = he/she/it is very lazy

Because öğrenci comes right after o, the meaning is that student, not he/she.

Turkish does not have articles like English a/an/the.

So öğrenci can mean:

• student
• a student
• the student

The exact meaning depends on context.

In this sentence, o helps make it specific:

• o öğrenci = that student

If you wanted to say a student, you could use:

• bir öğrenci = a student / one student

Çok means very here.

So:

• çok tembel = very lazy

In Turkish, words like çok usually come before the adjective they modify, just like very in English:

• çok güzel = very beautiful
• çok zor = very difficult
• çok tembel = very lazy

Be careful: çok can also mean many / much in other contexts.

For example:

• çok kitap = many books
• çok su = a lot of water

Turkish sentence order is often different from English.
A very common pattern in simple statements is:

• subject + description

So here:

• O öğrenci = the subject
• çok tembel = the description

That gives:

• O öğrenci çok tembel.

This final position for the main descriptive part of the sentence is very natural in Turkish.

No, not by itself in this sentence.

Because o is directly followed by öğrenci, it forms a noun phrase:

• o öğrenci = that student

If you want to say He/she is a student, you would say:

• O öğrenci.

That is a complete sentence on its own, where:

• o = he/she
• öğrenci = student

So context and sentence structure matter a lot.

Compare:

• O öğrenci çok tembel. = That student is very lazy.
• O, öğrenci. = He/she is a student.
  (Usually in speech, context would make this clearer.)

Grammatically, Turkish sometimes allows subjects or determiners to be omitted when the context is clear, but:

• O öğrenci çok tembel. = That student is very lazy
• Öğrenci çok tembel. = The student is very lazy / Student is very lazy

Without o, the sentence becomes less specific. You lose the clear meaning of that student.

So if you specifically mean that student, keep o.

In this sentence, tembel is an adjective meaning lazy.

But Turkish adjectives can often also function like nouns depending on context.

For example:

• tembel öğrenci = lazy student
• Tembeller çalışmaz. = Lazy people do not work.

So Turkish words can be flexible. Here, though, tembel is clearly describing öğrenci.

No—Turkish adjectives do not agree with nouns in gender or number the way they do in many European languages.

So tembel stays the same:

• tembel öğrenci = lazy student
• tembel öğrenciler = lazy students

Also, Turkish nouns do not have grammatical gender, so there is no masculine/feminine adjective change.

That makes adjective use simpler than in many languages.

Öğrenci is pronounced roughly like ur-en-jee or eu-ren-jee, but the exact sound is important:

• ö is like the vowel in German schön or French peu
• ğ usually does not sound like a hard consonant in modern Turkish; it often lengthens or smooths the vowel before it
• c in Turkish is pronounced like English j in jam

So öğrenci is approximately:

• öh-ren-ci
  or more smoothly
• ö-ren-ci

The ğ is one of the trickiest Turkish letters for English speakers, because it often has little or no direct consonant sound.

No—O öğrenci çok tembel is already fully correct.

Adding -dir gives:

• O öğrenci çok tembeldir.

This can sound:

• more formal
• more textbook-like
• more emphatic
• more like a general assertion

In everyday spoken Turkish, people usually prefer:

• O öğrenci çok tembel.

So tembeldir is not more correct, just different in tone.