The verb ending already shows who does it, so the subject pronoun (аз, ти, той...) is usually left out. Keep it only for emphasis or contrast.

The short words се, ми, ти, го and съм can't start a sentence. When the pronoun goes, they don't disappear, they move after the verb.

• "Аз се казвам Иван." → "Казвам се Иван." (My name is Ivan.)
• "Аз ще отида на кино." → "Ще отида на кино." (I will go to the cinema.)
• "Ти се смееш много." → "Смееш се много." (You laugh a lot.)
• "Той ми даде книгата." → "Даде ми книгата." (He gave me the book.)

Never start with the short word: "Се казвам Иван" and "Ми даде книгата" are wrong.