за- for Starting an Action

1. The meaning. За- + an imperfective verb can give a perfective verb that means "start doing":

• пея → запея (start singing), плача → заплача (burst into tears)
• валя → заваля (start raining), смея се → засмея се (burst out laughing)
• говоря → заговоря (start talking), танцувам → затанцувам (start dancing), свиря → засвиря (start playing music)

2. It is perfective. These verbs name the single moment the action begins, so they are most common in the aorist, after да and in the future: Детето заплака. (The child burst into tears.) Ако завали, ще се приберем. (If it starts raining, we will go home.) On its own in the present, a perfective verb is not used; see perfective and imperfective aspect.

3. The imperfective partner. For repeated beginnings, use the imperfective form: заплаквам, запявам, завалява. Щом чуе тази песен, запява. (Whenever she hears this song, she starts singing.)

4. За- or започвам да? Both are correct. За- is shorter and suggests a sudden start: Всички запяха. (Everyone burst into song.) Започнаха да пеят is neutral. After започвам the verb must be imperfective: започна да вали.

5. Other meanings of за-. За- also makes ordinary perfectives with other meanings: затворя (close), запиша (write down), закъснея (be late). Only some verbs, mostly of sound, weather, emotion and movement, get the "start" meaning. See aspect pairs formed with a prefix.