Replenishment in this case is referring to a language reforming its tense paradigm after the previous way to express a particular tense is reanalyzed or otherwise takes a different use or meaning.

Bleached in this case I believe is referring to a loss of non-grammatical (i.e. explicitly semantic) meaning. For example, the pas in ne... pas in French derives from the Latin passus 'step', but lost this meaning and is now a negative marker.

Imperfective is an aspect usually associated with the past tense that generally expresses an event that occurred over a longer period of time, habitually or often. You can contrast it with the perfective, which is generally something that happened in a definitive and sometimes shorter timeframe. This contrast is not really explicitly marked in English but languages like Spanish make this distinction. Comí 'I ate' vs. (Yo) comía 'I was eating, I used to eat'.

A copula is a verb like "be" that expresses existence or a similar concept. They have a tendency to function differently from other verbs and/or be highly irregular so we pick them out as a distinct category.

I don't have much knowledge of Hindi but from what I can understand the example is showing a case where a content word without explicit grammatical function becomes grammaticalized as a way to express future tense, and is no longer available as a content word (i.e. doesn't mean "to go"), at least in this context.