Alright so this may not be the best place to ask this, but it's something I've been confused on.
When you want to express "to want," you use tá, of course, for example "tá uaim a scriobh" for "I want to write."
But when you say you "know" something, you also use tá ("the knowledge is at me"), "tá a fhois agam . . ." . . . thus it's not a true verb.
So how would you express "I want to know . . ." ? Would you use "eolas" instead?
There are several ways of expressing 'I want to write': Teastaíonn uaim scríobh, Is mian liom scríobh, etc. The a is only used before the verbal noun following an object, e.g. Teastaíonn uaim litir a scríobh, Is mian liom leabhar a cheannach, etc.
The a in 'a fhios' is actually a possessive adjective ('its'/'his'). Tá a fhios agam literally means 'Its knowledge is at me', i.e. 'I have its knowledge'.
'I want to know' would be Teastaíonn uaim a fhios a bheith agam, Is mian liom eolas a bheith agam air, etc.
No. This is a question of idiomatic usage - the Irish equivalent of "I know" and "I don't know" use a fhios, and if you leave out the possessive adjective, you're no longer using an idiomatic phrase that people understand as the equivalent of "I know", you're now just saying "I have knowledge", without any indication that the knowledge that you claim to have is actually relevant to the topic at hand.
Incredible observation skill. Answer is, that "deir" is exception to the rule, scroll down on page to section "verbs".
lenition with verb after a direct relative particle a (except tá, deir): an teach a thógfaidh sé = the house that he wants to build
The "a" causes lenition. It's a preposition, and it's used here in a relative clause - you can expand the sentence to mean "what is it that they need?", and "a" is essentially the "that".
(There are a number of different "a"s in Irish, and you need to recognize what role the "a" is playing to decide which rules apply to it).