"I need eighteen now."
Translation:Feumaidh mi ochd deug a-nis.
I understand the question - why isn't it 'Tha mi a' feum(or something) ochd deug a-nis', in the same way you would say 'Tha mi ag iarraidh etc. '
The reason is (I think!) that 'feum' - to need - is an irregular verb with no present participle/verbal noun. 'Feumaidh' is a future form of the verb: literally 'I will need' , and as you know, the future is often used in place of the present in Gaelic.
I think the course creators are gradually introducing other forms of verbs, by feeding in a few examples. We get things like "Stad, Iain!" and "Chì mi a-rithist thu" for example. Eventually we'll have to tackle the "I heard" as well as the "I was hearing" constructions, presumably in the next iteration of the course.
Bha mi a' cluinntinn - I was hearing
Chuala mi - I heard