"Islam is a big religion."
Translation:Crefydd fawr yw Islâm.
One of the Welsh translation options given is 'mae Islam yn grefydd fawr' - why is 'yn' used here before a noun? Is it optional?
No, it is required to link mae and other verb-forms of bod to any noun or adjective. (Note the exception of dw i eisiau/angen, etc, which is an exception to the usual pattern, as pointed out in the notes.
(A more grammatical but rather unusual pattern with eisiau is, for example, Dw i ag eisiau arian (literally 'I am with a want of money'). You can find examples if you do a web search for "ag eisiau" with the double quotes. The ag is hardly seen in colloquial usage. The usual more formal form is Mae angen/eisiau arian arnaf i.)
Thanks for your comprehensive reply. I feel a bit embarrassed to have asked about something that is clearly so basic but I had got it into my head that 'yn' preceded only adjectives.
Don't apologise for asking something "so basic". The structure of Welsh sentences is pretty different to English and when to use yw or mae and little words like yn can be difficult to get your head round. It comes eventually with explanation and practice. Dal ati!