Short (enclitic) form - the everyday form

The short form is unstressed: it cannot stand alone or begin a sentence. It normally sits in the second position of the clause, leaning on the word before it.

„Ја сам уморан.“ (I am tired.) and „Уморан сам.“ (Tired, I am.) are both correct; „Сам уморан.“ is wrong because the enclitic сам cannot start a sentence.

Long (stressed) form - emphasis and short answers

The long form is stressed and can stand alone: use it to answer a yes/no question with one word - Јесам. (Yes, I am.) - and for emphasis: „Ја јесам студент.“ (I really am a student.)

The long form also combines with the question particle ли: „Јеси ли уморан?“ (Are you tired?)

Negation

The negative present is a single word beginning with ни-: нисам, ниси, није, нисмо, нисте, нису. Do not write it as two words: не сам is wrong.