'Progressive aspect' refers to a modification of the meaning of a verb or compound verb to indicate something that's ongoing. Thus 'Snámhainn mé' is in the simple present ('I swim'), while 'Tá mé ag snámh' is in the present progressive ('I am swimming').
'Areal feature' refers to a language feature (such as the progressive aspect) that is common to the languages of a particular area. The progressive aspect is a language feature that's common in the languages of Great Britain and Ireland, but its uncommon elsewhere in Europe. For instance, French fails to distinguish it and instead "Je nage" used to express both sense depending on context.
Thanks. But I think we're talking past each other. I have seen "ta se snamh" (or, at least, its equivalent) without the "ag" which, my understanding is, turns it from "He is swimming" into "he swims." I have also seen "Snamheann se" (or, at least, it's equivalent). What's the difference between those two phrases (if any), and (if there's no difference in meaning) are they used in different circumstance? That was my question.
I think I see what you are asking now!
Any verb can (at least in theory) exist in either form:
- Itheann sé feoil He eats meat
- Tá sé ag ithe feola He is eating meat
- Ritheann sé sa pháirc He runs in the park
- Tá sé ag rith sa pháirc He is running in the park
Irish does not have infinitives: the word "snámh" in Tá sé ag snámh is called a verbal noun. The direct translation of Tá sé ag snámh would be "He is at swimming" - in other words, he is currently engaged in the act of swimming. There is a whole skill on verbal nouns later on in the course.