Учитель похвалил мою внимательность, но сказал, что я порой пишу слишком небрежно.

Because the subject is учитель, which is a grammatically masculine noun.

In the past tense, Russian verbs agree with the subject in gender and number:

• он похвалил, сказал — masculine
• она похвалила, сказала — feminine
• они похвалили, сказали — plural

So with учитель, you normally get похвалил and сказал.

If you wanted to make it clearly female, you would often say:

• Учительница похвалила...

In real life, some speakers may use учитель сказала when talking about a woman, but for learners, the safest rule is: past-tense verbs usually match the grammatical gender of the noun.

Because внимательность is the direct object of похвалил.

The verb похвалить takes the accusative case:

• похвалить кого? что? — to praise whom? what?

So:

• моя внимательность = nominative, the subject form
• мою внимательность = accusative, the object form

Notice that the noun внимательность itself does not visibly change here, but the possessive adjective does:

• моя
• мою

That is why the case change is easy to spot in мою.

This is a question of verbal aspect.

• похвалил is perfective
• хвалил is imperfective

Here, похвалил shows a single completed act: the teacher praised the student once, in that situation.

So the sentence presents two completed speech actions:

• the teacher praised
• the teacher said

If you used хвалил, it would sound more like repeated praise, a process, or background information rather than one completed event.

Because the sentence is talking about a general habit or recurring behavior, not one finished piece of writing.

• пишу = I write / I am writing
• написал = I wrote / I have written something completed
• написываю is imperfective too, but it is much less natural here

The adverb порой means sometimes / at times, which strongly suggests a repeated tendency. That fits the simple imperfective пишу very well:

• я порой пишу слишком небрежно = I sometimes write too carelessly

Russian often uses the imperfective present for habits and general tendencies.

Порой means sometimes, at times, or from time to time.

It is a perfectly normal word, but its tone is a little more literary or polished than the very common иногда.

Compare:

• иногда — sometimes
• порой — sometimes / at times, slightly more expressive or literary

In this sentence, порой works well because it sounds natural in a teacher’s comment.

You could replace it with иногда without changing the basic meaning much:

• ...что я иногда пишу слишком небрежно.

They are related, but they are not the same.

• внимание = attention
• внимательность = attentiveness, carefulness, being observant

In this sentence, внимательность is better because the teacher is praising a personal quality or trait, not just a moment of paying attention.

So:

• внимание is often the act or state of attention
• внимательность is a character trait or consistent quality

That is why похвалил мою внимательность sounds natural here.

Небрежно is an adverb meaning carelessly, sloppily, or not neatly.

It comes from небрежный:

• небрежный почерк — sloppy handwriting
• писать небрежно — to write carelessly / sloppily

In this sentence, it most likely refers to the way the person writes by hand: handwriting, neatness, or care in written work.

Since it is an adverb, it describes the verb пишу:

• пишу как? — небрежно

Слишком means too or overly.

It intensifies the adverb небрежно:

• небрежно — carelessly
• слишком небрежно — too carelessly / overly carelessly

So the teacher is not just saying the writing is careless sometimes, but that it is more careless than it should be.

This is a common pattern in Russian:

• слишком быстро — too quickly
• слишком громко — too loudly
• слишком поздно — too late

Because что introduces a subordinate clause, similar to that in English.

• сказал, что... = said that...

So the sentence structure is:

• Учитель похвалил...
• но сказал, что...

The part after что is the content of what was said:

• что я порой пишу слишком небрежно

In English, that is often optional:

• He said that I sometimes write too carelessly.
• He said I sometimes write too carelessly.

In Russian, что is normally expressed here.

Yes, it could sometimes be omitted in conversation, but including it is normal and clear.

• ...сказал, что я порой пишу слишком небрежно — fully explicit
• ...сказал, что порой пишу слишком небрежно — possible, especially in informal speech if the subject is obvious

Russian often omits pronouns when the verb ending already shows the subject, but in subordinate clauses speakers frequently keep the pronoun for clarity, rhythm, or emphasis.

So in this sentence, я sounds completely natural and standard.

The basic meaning would stay the same if you changed the word order a bit, but the emphasis would shift.

The given version is very natural:

• Учитель похвалил мою внимательность, но сказал, что я порой пишу слишком небрежно.

Other possible versions:

• ...что я слишком небрежно пишу.
• ...что порой я пишу слишком небрежно.
• ...что я пишу небрежно порой. — less natural in neutral speech

Russian word order is more flexible than English, but not random. The original sentence sounds smooth and neutral, with:

• порой placed before the verb
• слишком directly before небрежно

That is a very typical arrangement.

Both commas are required.

1. Comma before но
   Но means but and joins two clauses:

• Учитель похвалил мою внимательность
• но сказал...

2. Comma before что
   Что introduces a subordinate clause:

• сказал, что я порой пишу слишком небрежно

So the punctuation reflects the sentence structure:

• main clause
• coordinating conjunction но
• another main clause
• subordinate clause introduced by что