Learn Hardcore Turkish: Pazartesi sabahları genellikle biraz gergin oluyorum. - On Monday mornings I usually feel a bit tense.

Sabahları literally comes from sabah (morning) + -lar (plural) + a form that historically looks possessive, but in modern Turkish here it works as an adverbial time expression.

In practice:

sabah = morning
sabahları = in the mornings / on mornings (generally)

So Pazartesi sabahları means “on Monday mornings (as a habit, regularly)”, not one specific Monday morning. The plural helps express repetition / habitual action, like English “Mondays” or “on Monday mornings”.

Why is there no -de/-da (meaning “in/on/at”) after Pazartesi or sabah?

In Turkish, time expressions used as adverbials often do not need the locative ending -de/-da.

Pazartesi sabahları = on Monday mornings (already functions as a time adverbial)
You don’t need Pazartesi sabahlarında here; that would sound heavier and is usually unnecessary.

Compare:

Gece çalışıyorum. – I work at night.
Yazın tatile gidiyoruz. – We go on vacation in summer.

Same idea: bare time words (sometimes with plural or other endings) can directly serve as “when?” information without -de/-da.

What does oluyorum mean exactly? Why not just say gerginim?

Oluyorum is from the verb olmak (to be, to become):

ol- (root “be/become”)
-uyor- (present continuous)
-um (1st person singular “I”)

So oluyorum = I am becoming / I get / I am (in the process of being).

In your sentence:

Pazartesi sabahları genellikle biraz gergin oluyorum.
On Monday mornings I usually get a bit tense.

Using olmak with an adjective often means “to become X / to get X”, especially for states that arise or come over you.

gerginim = I am tense (state, description)
gergin oluyorum = I become / I get tense (it happens, it kicks in)

Here, because it happens regularly at a certain time (Monday mornings), oluyorum fits very well.

So is gerginim wrong here, or just different?

It’s not wrong, but the nuance changes.

Pazartesi sabahları genellikle biraz gerginim.
Sounds like: On Monday mornings I’m usually (already) a bit tense – a more static description.
Pazartesi sabahları genellikle biraz gergin oluyorum.
Sounds like: On Monday mornings I (tend to) get / become a bit tense – emphasizes the process or tendency to become tense at that time.

In natural speech, oluyorum here sounds a bit more dynamic and typical for describing a mood that arises on those mornings, rather than a fixed characteristic.

Why is the tense oluyor (present continuous) used when the English translation is “I usually get tense”, not “I am getting tense”?

Turkish present continuous (-yor) is also used for:

Habits and repeated actions, especially when you have adverbs like
genellikle (usually), sık sık (often), hep (always), etc.

So:

Genellikle kahve içiyorum. – I usually drink coffee.
Her sabah yürüyüş yapıyorum. – I (usually) go for a walk every morning.

Similarly:

Pazartesi sabahları genellikle biraz gergin oluyorum.
= On Monday mornings I usually get a bit tense.

English prefers the simple present (I usually get tense), while Turkish often uses -yor in this kind of habitual context.

What is the role of genellikle here, and where can it go in the sentence?

Genellikle means “usually, generally, most of the time”. It’s an adverb of frequency.

In Turkish, adverbs like this are quite flexible in position. All of these are natural, with very similar meaning:

Pazartesi sabahları genellikle biraz gergin oluyorum.
Genellikle pazartesi sabahları biraz gergin oluyorum.
Pazartesi sabahları biraz gergin oluyorum genellikle. (more colloquial, end-position)

Word order mostly affects emphasis / rhythm, not the basic meaning. The version you have is neutral and very common: [time] + genellikle + (rest of clause).

What does biraz do here? Is it necessary?

Biraz means “a bit, a little (somewhat)”. It softens the adjective gergin:

gergin = tense / nervous
biraz gergin = a bit tense / a little nervous

Is it necessary? No, but it changes the intensity:

... genellikle gergin oluyorum.
= I usually get tense (stronger).
... genellikle biraz gergin oluyorum.
= I usually get a bit tense (softer, more moderate).

Grammatically, both are fine; biraz just adds nuance.

Why is it Pazartesi sabahları and not Pazartesileri sabah or something similar?

You have a few natural options in Turkish to say “on Mondays / on Monday mornings”:

Pazartesi sabahları – on Monday mornings (most natural here)
Her pazartesi sabahı – every Monday morning
Pazartesileri – on Mondays (more general; often without sabah)

Your sentence focuses specifically on the morning, so:

Pazartesileri gergin oluyorum. – On Mondays I get tense. (the whole day, more general)
Pazartesi sabahları gergin oluyorum. – On Monday mornings I get tense. (time is more specific)

Pazartesi sabahları is the cleanest way to say “on Monday mornings” as a repeated time frame.

How would I say this sentence in a negative or question form?

You keep the structure, just change oluyorum:

Negative

Pazartesi sabahları genellikle hiç gergin olmuyorum.
On Monday mornings I’m usually not tense at all.
Pazartesi sabahları genellikle gergin olmuyorum.
On Monday mornings I usually don’t get tense.

Form: ol-ma-yor-um (become-not-PRS.PROG-1SG)

Yes/No question

Pazartesi sabahları genellikle biraz gergin oluyor muyum?
Do I usually get a bit tense on Monday mornings?

Notice the question format:
oluyor muyum? → verb + mi (question particle, separated) + personal ending.

Can I move biraz or gergin around, or do they have to stay together?

Biraz modifies gergin, so they usually stay together as a small phrase: biraz gergin.

Most natural orders:

Pazartesi sabahları genellikle biraz gergin oluyorum.
Pazartesi sabahları biraz gergin oluyorum genellikle. (spoken)
Genellikle pazartesi sabahları biraz gergin oluyorum.

Unnatural or wrong would be splitting biraz far from gergin in a way that breaks the phrase, e.g.:

✗ Pazartesi sabahları biraz oluyorum gergin.

So keep biraz gergin together; then you can move that block around with respect to other adverbs if needed.