The special verbal form you refer to is called the Copula. It’s present tense form is ‘is’ and the one for the past tense/the conditional is ‘ba’. For example:
Is fear é – He is a man
B’fhear é – He was a man/He would be a man
For a full discussion of the Copula, check out the Nualeargais.ie website. It’s pretty good and comprehensive.