aspect in English

The English past tense is aspectually neutral. A single form, sang, covers both a bounded event and an unbounded one, and the reading is settled by the surrounding words or left open. This is the largest single divergence from a language whose past inflection forces the choice, and it is why the state here is periphrastic rather than explicit: no inflected verb form of English carries an aspectual value, in the past or anywhere else.

What the language has instead is two constructions, each an auxiliary plus a participle. The progressive is a form of be plus the gerund participle, and it presents the event as in progress at the reference time. The perfect is a form of have plus the past participle, and it presents the event as complete with its consequences holding at the reference time. The two are independent and combine freely, so a clause can be progressive, perfect, both in either order of nesting, or neither, and the four possibilities are all grammatical with different meanings. The habitual has no dedicated construction: the simple present carries it by default, and used to plus the plain form carries it in the past.

The exponent is the construction and not the suffix, which is the reason for the classification. The gerund participle appears in many places that are not progressive at all, and the past participle appears in the passive as readily as in the perfect, so neither ending means an aspectual value on its own. Only the pairing of auxiliary with participle does. The obligatoriness is recorded as conditioned rather than obligatory because the requirement holds under a stated condition and not generally: a dynamic event ongoing at the moment of speaking must take the progressive, and using the simple present there is ungrammatical on that reading, while a stative verb is refused the progressive and a past event is under no aspectual obligation whatever.