When to put a before "a" before something??
Im new to this and im confused as to when you put "a" in front of some thing? for example you would say " a phiuthar" but for other things there is no "a" for example "athair", why is this and how do i know when to use "a"?
It's in the tips & notes. https://duome.eu/tips/en/gd
It's called the vocative case and is used when addressing someone, so when you speak to someone you add the 'a' in Gaelic (and lenite) and when you talk about someone you don't. Unless it's a noun starting with a vowel, then you drop it. :)
The particle a is used for different things.
1- It is used before a verbal noun in certain sentence structures. This is pretty complex grammar and quite different from English grammar, so don't worry to much about it at this stage--just be aware of it. Know that it lenites the verbal noun when possible, but it will be dropped before a verbal noun that begins with a verb.
Feum mi a thighinn dhachaigh - I can come home. A bheil thu airson a thighinn dhachaigh? - Do you want to come home?
(When it gets dropped) A bheil thu airson ithe? - Do you want to eat?
2 - It is used in the vocative case, when addressing someone directly, like with a greeting or a command:
Seall air, a Chaluim - Look at it, Calum.
When you address someone who is male, you lenite when possible. When you address someone who is female, you do not lenite.
Madainn mhath dhuibh, a Màiri. - Good morning, Mary.
And before names that start with vowels, it is dropped:
Thig a-staigh, Anndra - Come in, Andrew.
3 - The possessive 3rd person singular pronoun, the equivalent of his/hers in English. If it's "his", it always lenite when possible. When it's "hers", it doesn't lenite. (Note that for "its" it is usually better to use the genitive case of the noun--which you will learn later--instead of a possessive pronoun, but when the possessive pronoun is used, it takes the gender of the noun it's replaces, or the conjugated pronouns that I talk about below.)
a bhiadh - his food a biadh - her food
This is like the example you gave -
a phiuthar - his sister
If it was "her sister" it'd be "a puithar."
With the masculine possessive pronoun, it will be dropped before nouns that begin with a vowel:
athair - his father (yes, it looks just like "a father" but you should be able to understand which one it is from context)
But with the female possessive pronoun, you must insert a h-:
a h-athair - her father
Lastly, with nouns that cannot be lenited, you'll have to rely on context to know if it's "his" or "her"
a leapaidh - his bed
a leapaidh - her bed
There is another way to express "his" and "hers" that is clear about the possessor's gender in this case, and it's used most commonly with things that aren't inherently belonging to or more exclusive to that person (like body parts, family members and intimate possessions) but with things that are more transitory or can change ownership more easily. This uses the definite article before the owned object and then the conjugated pronouns agam/agad/aige/aice/etc.
an leabhar agam - my book (lit. the book at me)
an leabhar agad - your book (lit. the book at you)
an leabhar aige - his book (lit. the book at him)
an leabhar aice - her book (lit. the book at her)
Of these three ways you can use "a" the most important for you to know right now is 3, the possessive pronoun. Get that down pat. Here's a webpage that give you some other examples: https://gaelicgrammar.org/~gaelic/mediawiki/index.php/Possessive_Pronouns You will need to memorize these pronouns, when they lenite or don't, and when there are special changes before a noun that starts with a vowel.
Also, beginners will need to know 2, the vocative case, as well, but that's at least very simple.
But like I said about 1, with the verbal noun, that's a tricky thing for English speakers to get handle on so just be aware of that for now so when you see a sentence with it, it's not completely baffling. But you don't need to master that just yet.