El pluscuamperfecto o el pluperfect_

Question

Hi.... how do you choose between habia (etc) and hubiera/hubiese (etc).  What are these called? Thanks in advance

Answer

Hola Wendy
había and hubiera are both from the auxiliary verb "haber" in Spanish. The verb haber is used to either say "there is.., there was..., there has been..., there will be... etc" in which case you just need the form of haber in the tense you want in the 3rd person singular:

Había mucho tráfico.
There was a lot of traffic.
Habrá mucho tráfico.
There will be a lot of traffic.

The verb haber is also used for perfect tenses, for example: he has worked, she had worked, she will have worked, etc. In this case, we use haber in the tense you want and then the participle of the main verb:

Elena ha trabajado.
Elena has worked.
Elena había trabajado.
Elena had worked.
Elena habrá trabajado.
Elena will have worked.

We also have all these tenses in the subjunctive mood, and this is where hubiera/hubiese come along. When we form the Pluperfect subjunctive of a verb, we use these. The pluperfect indicative would be formed with había plus participle, while the pluperfect subjunctive is formed by hubiera/hubiese plus participle. There are different situations when we use the subjunctive tenses but to simplify it a bit, you'd be using the pluperfect subjunctive in hypothetical sentences talking about the past. For example:
A statement could be: "Elena había trabajado en ese restaurante." (Elena had worked in this restaurant.)
And then we could use a hypothetical sentence saying for example:

Si Elena hubiera trabajado en ese restaurante, habría ganado mucho dinero.
If Elena had worked in that restaurant, she would have earned lots of money.

To help you understand it a bit more you can go to this kwiziq lesson.
You can also see here a table with the most common names of all tenses in Spanish, both in Spain and Latin America. 
Un saludo
Inma