This is actually the future (or maybe non-past would be a better term) form (that may be used both for future meaning, and habitual present, like English present tense).
The forms of bi to be, the substantive verb, are as follows (first the independent form –used as the beginning of a sentence, the second dependent – used after negative, interrogative, etc. particles, like cha(n), nach, gu, etc.):
- tha and eil (lenited), bheil (after a in questions) are the present forms: tha mi… I am…, chan eil mi… I am not, a bheil mi…? am I? (at the moment)
- bha and robh are past: bha mi… I was…, cha robh mi… I was not…
- bidh and bi are future (non-past): bidh mi… I will be or I do (be), I usually am, cha bhi mi… I will not be… or I don’t…, I usually am not…
Just as in Old English and Irish, there are two different verbs for 'to be' and they have a lot of confusing forms beginning with b as if the two verbs got muddled at some time in the past. Add the forms that are lenited and start with bh. Modern English only has one verb, but it is still a muddle, with some forms in b (such as be) and some in w (such as was) which is effectively a variant of bh.
So we have bu in Gaelic, the past of is, meaning 'was', and bhí, which looks like bhi but is in fact the Irish for bha, 'was'.
So the answer to why this appears to be past tense is that it is very confusing, especially if you know any other languages that use b words for these verbs.
You just have to learn the correct forms for Gaelic carefully.
beginning with b as if the two verbs got muddled at some time in the past
The b-forms of the copula and bi both come from the PIE *bʰuH-, the copula is comes from *h₁ésti (like English is), tha comes from Old Irish at·tá which comes from PCelt. *tāti ‘to stand’ (there is a dog was there stands a dog) while the dependent (bh)eil comes from OIr. fil, itself from PCelt. *weleti ‘to see’ (so there is not a dog was originally see no dog there, we are not there was don’t see us there, or sth like that – and because of that the subject of fil in OIr. was still in accusative – since it was an object of seeing – while the subject of the independent at·tá and other forms was in nominative).
Thank you for the detail. So tha is related to the substantive in Spanish and Portuguese esta/está. (I had guessed but did not know).
And I forgot about fil. So basically a completely different verb, with a different initial sound and a different meaning is adopted as a suppletive form, then undergoes a different mutation (nasalization) so it now sounds like bh (bhfuil in Irish) then gets spelt to look like a lenited b in Gaelic as we don't have nasalization or the spelling bhf. So simple.