English could points in two directions, and Spanish keeps them apart. I couldn't do it yesterday is a past. But I could do it tomorrow is really would be able.
Podíamos · No podía hacerlo porque no tenía el tiempo · No podía hacerlo ayer
Put haber into the line past and you can say I had eaten rather than I have eaten. For this, you always use the line past. Always.
había + the version after haber
Había comido · Ya había comido · Ya había cenado · Yo lo había perdido
When to use had rather than have needs no discussion: it works exactly as in English, so you just translate the thought.
Ya tends to go at the beginning, which is the least emphatic and most natural spot.
The student reaches for "cenía" and "cenaría" when trying to translate dined. The teacher cuts it off: what you want is not a translation of dined.
Deciding between the line and the point is a decision about context, not about matching whichever one English used.
Quería is a good example. English says he wanted, not "he was wanting", but the line past is fine in Spanish: wanting something is usually an ongoing feeling rather than an instant.
Él quería cenar conmigo, pero yo ya había cenado · Lo buscaba porque yo lo había perdido
The student means to say "habería" but does not quite pronounce the e. That is precisely how the contraction happened, and Spanish has formalised it in the spelling: h-a-b-r-í-a.
Lo habría perdido · Me habría perdido
Only a few verbs contract this way.
Asked for he was looking for it, the student produces "buscía". The fix is the habit you already have.
buscar is an -ar verb → buscaba
Lo buscaba · Me quedé