this can't be just 'the tiger in the forest'? presumably because the tiger isn't actually 'verb'-ing?
There are two ways of translating this sentence to English.
Either as 'A tiger is in the forest'
Or as 'There is a tiger in the forest'
'Mae' is the form of 'bod' the verb 'to be' which is used for the third person singular, it is also the word used for the expression 'There is/There are'