Τις μέρες που αντιμετωπίζω πολλή πίεση στη δουλειά, νιώθω πιο πιεσμένος και μιλάω λιγότερο.

Because τις μέρες is in the accusative plural, and Greek often uses the accusative to express time.

Here, τις μέρες που... means something like on the days when... or during the days when....

So:

• οι μέρες = the days as a subject
• τις μέρες = the days / on the days as an object or time expression

In this sentence, it is not the subject of the verb, so οι μέρες would not fit.

Here που introduces a relative clause and connects it to τις μέρες.

So:

• τις μέρες που αντιμετωπίζω πολλή πίεση
  = the days when I face a lot of pressure

Even though English would usually say when, Greek very often uses που in this kind of structure.

A very literal breakdown would be:

• τις μέρες = the days
• που αντιμετωπίζω... = that / when I face...

The present tense is used because the sentence describes a repeated or habitual situation, not a single completed event.

Τις μέρες που αντιμετωπίζω πολλή πίεση στη δουλειά... means:

• On days when I am facing a lot of pressure at work
• or On days when I face a lot of pressure at work

Greek present tense often covers both:

• general habits
• repeated situations
• what is happening in those kinds of circumstances

So the present is the natural choice here.

Because πίεση is a feminine singular noun, and πολλή agrees with it.

So:

• πολλή πίεση = a lot of pressure
• πολύ would be wrong here if you are using it as an adjective with πίεση

Compare:

• πολλή δουλειά = a lot of work
• πολλός χρόνος = a lot of time
• πολύ νερό = a lot of water

But πολύ can also be an adverb, for example:

• δουλεύω πολύ = I work a lot

So in this sentence, since it modifies the noun πίεση, it must match the noun: πολλή.

Στη is the contracted form of σε τη.

So:

• σε + τη δουλειά → στη δουλειά

It means at work or at the workplace, depending on context.

This kind of contraction is very common in Modern Greek:

• στο = σε το
• στη = σε τη
• στην = σε την
• στους = σε τους
• στις = σε τις

Here δουλειά is feminine, so στη δουλειά is the expected form.

Πιεσμένος is an adjective meaning pressured, stressed, or under pressure. It agrees with the person speaking.

Since the subject is an implied I, the form tells us the speaker is masculine singular:

• πιεσμένος = masculine singular
• πιεσμένη = feminine singular

So a woman would normally say:

• νιώθω πιο πιεσμένη

The word πιο means more, so:

• πιο πιεσμένος = more pressured / more stressed

Because Greek often drops subject pronouns when they are already clear from the verb ending.

Here the verbs show the subject clearly:

• αντιμετωπίζω = I face
• νιώθω = I feel
• μιλάω = I speak

So adding εγώ is usually unnecessary unless you want emphasis or contrast.

For example:

• Εγώ νιώθω πιο πιεσμένος
  = I feel more stressed
  with extra emphasis on I

Because λιγότερο here is an adverb, not an adjective.

It modifies the verb μιλάω:

• μιλάω λιγότερο = I speak less

Since it is functioning adverbially, it does not change to match a noun.

Compare:

• μιλάω λιγότερο = I speak less
• έχω λιγότερα προβλήματα = I have fewer problems

In the second example, λιγότερα is an adjective and agrees with προβλήματα. In your sentence, it is simply modifying the action of speaking.

Yes. Both mean I speak.

• μιλάω is very common in everyday Modern Greek
• μιλώ is also correct and may sound a bit more formal or concise, depending on context

So both of these are possible:

• μιλάω λιγότερο
• μιλώ λιγότερο

In normal conversation, μιλάω is extremely common.

Yes, Greek could also use όταν, but the nuance is a little different.

Your sentence:

• Τις μέρες που αντιμετωπίζω πολλή πίεση...

focuses on those days as a type of recurring situation: the days when...

A version with όταν would be:

• Όταν αντιμετωπίζω πολλή πίεση στη δουλειά...
• When I face a lot of pressure at work...

Both are natural, but τις μέρες που... is slightly more specific and descriptive, because it frames the idea around particular kinds of days.

The comma separates the introductory time clause from the main clause.

The structure is:

• Τις μέρες που αντιμετωπίζω πολλή πίεση στη δουλειά,
  introductory clause
• νιώθω πιο πιεσμένος και μιλάω λιγότερο.
  main clause

This is similar to English punctuation in a sentence like:

• On days when I’m under a lot of pressure at work, I feel more stressed and speak less.

So the comma helps show where the first idea ends and the main statement begins.

Because Greek, like English, can join two actions with και when they have the same subject.

Here the subject of both verbs is the same implied I:

• νιώθω πιο πιεσμένος = I feel more stressed
• και μιλάω λιγότερο = and I speak less

Greek does not need to repeat εγώ or restructure the sentence. This is a very normal and natural coordination of two actions.

In this sentence, πίεση most naturally means pressure in the sense of stress, work pressure, or mental strain, not physical pressure.

Greek often uses πίεση this way, especially in expressions like:

• πίεση στη δουλειά = pressure at work
• νιώθω πιεσμένος = I feel pressured / stressed

So the noun πίεση and the adjective πιεσμένος work together very naturally in this sentence.