Learn Hardcore Turkish: Derin nefes almak gerginliği azaltır.

Derin nefes almak: “to take a deep breath / deep breathing” (more infinitival)
Derin nefes alma: “the act/practice of deep breathing” (more activity-as-noun)

So derin nefes almak literally means to take a deep breath. In Turkish, you “take” a breath to inhale (nefes almak) and you “give” a breath to exhale (nefes vermek). As a general activity, nefes almak also means “to breathe.”

Why is the verb in the aorist form azaltır instead of the present continuous azaltıyor?

Azaltır (aorist) expresses general truths, habits, or regularly occurring effects. The sentence states a general fact: “Deep breathing reduces tension.” Azaltıyor would mean “is reducing,” describing an ongoing, specific situation right now, which is not the intent here.

Could I use azalır instead of azaltır?

No, that would change the meaning.

Azalmak = to decrease (intransitive: something decreases by itself) → gerginlik azalır = “tension decreases.”
Azaltmak = to reduce (transitive: someone/something reduces something else) → derin nefes almak gerginliği azaltır = “deep breathing reduces tension.”

You need the transitive azaltmak here because the subject causes the reduction.

Why does gerginlik take the accusative ending -i (gerginliği)?