Russian's single most distinctive grammar feature isn't a case or a gender — it's verb aspect, a category most English speakers have never had to think about explicitly before learning a Slavic language.
Nearly every Russian verb concept has two forms: an imperfective verb (for ongoing, repeated, or general actions) and a perfective verb (for a single, completed action). These are often entirely different-looking verbs, not the same verb with a different ending — you effectively learn verbs in aspect pairs from the start.
English tense answers "when" (past/present/future); Russian aspect answers a separate question — "was this completed, or ongoing/repeated?" — that applies independently of tense. A past-tense sentence can use either aspect depending on whether the speaker is describing a finished event or a repeated/ongoing one.
Because aspect is learned per verb pair rather than through a general rule, vocabulary study for Russian verbs typically means learning both members of a pair together from the beginning, rather than treating aspect as an advanced add-on.
A grammatical category distinguishing completed actions (perfective) from ongoing or repeated ones (imperfective), usually expressed through largely different-looking verb pairs rather than a shared verb with different endings.
No — tense answers "when" (past/present/future), while aspect answers a separate question about whether an action was completed or ongoing/repeated, and applies independently of tense.
In pairs, generally — since aspect is tied to specific verb forms rather than a general rule, learning both the imperfective and perfective form together from the start is the more practical approach.
It's expected to be introduced as core content given how central it is to Russian grammar, following the same 'teach real structure early' approach used in Tarc's existing courses.
See also: Russian Grammar · Russian Vocabulary · Learn Russian