For expressing an intention for the future (I am going to...) of course we use the construction: ir (conjugated) + a + infinitive. I have seen and used the present and imperfect tenses for this purpose-- Voy a comprar... Ibas a comprar... etc. Today a teacher told me that one could also use the simple future of ir in the "ir a" construction, eg. iré a comprar... This seemed completely weird to me and redundant; I asked her: Why wouldn't I just say Compraré...? So my question is: Which tenses of "ír" are permissible in the "ir + a + infinitive" construction? Thank you
You can use the ir a [inf] construction in any tense (iba/fui/iré a nadar), mood (que vaya/fuera/fuese/fuere a nadar) or even aspect (ha ido / está yendo a nadar), although some are certainly vastly more common than others.
While using it in the indicative future might seem redundant, it can express an ordering of future events, effectively like the difference between simple preterite and pluperfect.
That said, just because you have ir a [inf] doesn't mean you're indicating an future action. Especially in some forms (future, preterite) and always for others (imperative/exhortative subjunctive), it's not the periphrastic future construction, but rather plain ir with a prepositional phrase indicating purpose or intent (notice how Vete/andate/idos/váya(n)se a nadar works).