"Islam is a big religion."
Translation:Crefydd fawr yw Islâm.
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.)