"Is" is used to say that one object is another object or in a class of objects, i.e. I am a man, I am the postman
"Tá" is used to say that an object is in a state: I am in Cork, I am sick, I am here,I am going
Of course it gets more complicated but that is the basic distinction.
It's comparable to the distinction between the English predicate nominative and predicate adjective. "Is" in Irish basically equates something. When you say, "I am a man", you are simply equating yourself with man, so "Is" is used. However, if you say "I am in Cork", that sentence doesn't equate anything. It simply describes the subject, in terms of location. So, "Ta" is used.
So, I read all the comments, and I'd just like to get this clear in case I missed something. Sí and é and í are "she," "he," and "it" respectively. When saying "It eats," you have no option but to use "í." But if you see the word Sí or é, it could be synonymous with it? Like in English? I could theoretically say "it is eating," when I see a girl or a guy eating...just not very politically correct. :) Am I thinking along the right lines here or is there some rule that says when you use "Sí" as "she" or "it" for instance?
No, there is no "it" in Irish. " sí " means "she " and " sé " means "he " and "siad" means "they ", but with the special verb, the copula "Is", the forms are shortened to " í " for "she " and " é " for "he " and "iad" for "they ".
In English using it for he or she when you know the sex of the person would be rude, but we do start sentences out with "It is ...." which is not rude and which is similar to saying "this is...." or "that is..." When you say "it eats", you are usually talking about an animal or perhaps a monster. If you translate that to Irish, which pronoun you will use will depend on whether the Irish word for the animal is masculine or feminine. "Itheann sé." or "Itheann sí." https://www.duolingo.com/skill/ga/Basics-1 https://www.duolingo.com/skill/ga/Basics-2
When you use the copula "Is", it is like getting a warning "The following two words are = and refer to the same person or thing." So the word order is like VSS or "Verb + Subject Complement (or Predicate Nominative) = Subject" I consider it V+(PN)=S. All the other verbs use VSO order.