Participles

A' & Ag are shortened forms of Aig (at). A' is used before consonants & Ag is used before vowels (and before ag ràdh). If you can place the words - in the act of before an English word with a "ing" ending you need to put a' or ag before the Gaelic word.
Examples
a' dol - Going
ag obair - Working
a' tighinn - Coming
ag ràdh -saying

Tha mi a' dol dhachaigh - I'm going home
Tha iad ag obair anns an achadh - They are working in the field
Tha sinn a' tighinn - We are coming
Tha i ag ràdh gu bheil e fuar. She is saying it is cold