Negation of the perfect

In the last lesson you built the past out of two pieces: the little word sam, si, je, smo, ste, su and the l-participle. Bio sam na moru. — "I was at the seaside." To say that something did not happen, you keep the participle exactly as it is and swap that little word for its negative twin.

You have already met these negative forms in the very first lessons: they are the same nisam, nisi, nije, nismo, niste, nisu you use for "I am not". Nothing new to memorise.

The negative word comes first

This is the real difference, and it is a difference of position. Sam, si, je… are unstressed leftovers that lean on the word in front of them, so they can never start a sentence — that is why you say Bio sam u Grčkoj and never Sam bio u Grčkoj.

Nisam, nisi, nije… are full, stressed words. They lean on nothing, and they very naturally open the clause: Nisam bio u Grčkoj. The participle simply follows. You can also say Ja nisam bio u Grčkoj when you want to stress ja, but the short version with the negative first is what people say.

• Bio sam na plaži. → Nisam bio na plaži.
• Kupali smo se svaki dan. → Nismo se kupali svaki dan.
• Ona je ostala kod kuće. → Ona nije ostala kod kuće.

Never ne sam, ne si, ne je. The particle ne negates ordinary verbs (ne pijem), but the past auxiliary has its own fused negative, exactly like nemam for imam.

The participle still agrees

Negating changes only the auxiliary. The participle keeps agreeing with the subject in gender and number, so a man says nisam bio, a woman says nisam bila, a mixed or male group says nismo bili, and a group of women says nismo bile.

Saying no on its own

Because the negative form is a real word, it can stand alone as a whole answer. Da li si bila u Italiji? — Nisam. The affirmative cannot do that: there you need the long form, Jesam.

To correct someone who assumed the wrong thing, put the negative first and then say what really happened: Nismo išli na more, bili smo u planini. — "We didn't go to the seaside, we were in the mountains."