Translation:The suit costs forty-three pounds.
if 'punt' is feminine, why dont we use the feminine form of 3 or 4 (or both) here? is it because it's part of the larger number "43"?
That's an interesting question. The basic answer is that older number forms are disappearing from the language as it modernises, mainly as Welsh is used to teach Maths in Welsh medium schools. The old vigesimal system (similar to French) used feminine numbers, when appropriate, throughout. The most recent changes mean we only use the feminine numbers for 2,3 and 4.
The decimal system has never, as far as I know, used feminine numbers so that makes your suggestion correct, ie there is no feminine form of 43.
Does the decimal system also eschew mutations after higher numbers than 2, 3, maybe 6, 10?
Because otherwise pedwar deg tri phunt after the masculine (mutating) tri would be expected, wouldn't it?
A very good question. There are two completely correct ways of saying 'forty three pounds' in Welsh. The old vigesimal system gives 'Tair phunt ar ddeugain' (lit:- 'three pounds on two score), and the absolutely correct decimal way 'Pedwar deg tri o bunnoedd' (lit:-Forty three of pounds).
However in spoken Welsh, like any language, people look for shortcuts and so the grammatically incorrect form 'pedwar deg tri punt' will be commonly heard. Since this is the easiest way of saying it and the most useful form in speech we've used it in the course.
I think you mean Tair punt are ddeugain without mutation on punt?
I thought that feminine tair does not cause the following noun to mutate in Welsh (unlike [modern] Cornish: tri thas "three fathers (m.)" but also teyr fel "three balls (f.)").
On a different point: Why does this translate as the suit 'costs' 43 pounds? I tried 'cost' and was wrong but I don't understand why.
"The suit costs 43 pounds" - this is present tense.
"The suit cost 43 pounds" - this would be simple past tense. (Or it might be a present-tense verb appropriate for a plural subject, but "suit" is treated as a singular noun in English even though it is composed of multiple pieces of clothing such as a coat and pair of traousers.)
The Welsh sentence, starting as it does with Mae..., is in the present tense, so the past-tense translation is not appropriate here.