Tafsir of Al-Insan 76:7

The statement "They fulfill [their] vows" is a resumption of speech [isti’naf] introduced to explain the reason for which they are provided with this bliss. It contains a form of elaboration on what the term "the righteous" [al-abrar] signifies in summary. It was asked: "What do they do to attain such a high rank?" and the response was: "They fulfill [their] vows," and so on.

It is stated that this is a resumption for the purpose of clarification. Despite this, the shift was made from the imperative "fulfill" [awfu] to the imperfect tense [yufuna] to denote mental presence [istihdar] and to indicate continuity. Fulfilling a vow is a metonymy for performing all obligatory duties, as what is other than the vow is included by way of primary precedence [al-tariq al-awla]. The subtle indication here is that whoever fulfills what he has obligated upon himself, then the fulfillment of what Allah the Exalted has obligated upon him is more important and more deserving for him.

To treat this as a metonymy is what is required by what has been narrated from Qatadah, while Ikrimah and Mujahid held that it should be kept to its literal meaning—that is, if they vow an act of obedience, they perform it.

"...and they fear a Day whose evil..."—meaning its punishment—"...is widespread [mustatiran]," meaning it is prevalent, spread, and scattered throughout all regions, derived from the spreading of fire and the dawn [al-fajr]. It is more emphatic than "tara" (flew/spread), because an increase in the structure of the word indicates an increase in its meaning. It also signifies that which is sought [al-talab], for that which is sought is typically pursued with exaggeration. Describing them with this [trait] is an intimation of the excellence of their belief and their avoidance of sins.