This is the 715th most frequent Estonian word.
hakkasid
"Started" or "began" (past tense, 2nd person plural or formal you).
The word 'hakkasid' is used to indicate the beginning of an action, here referring to the activity 'exercising'.
Nad hakkasid varakult trenni tegema.
They started exercising early.
'Hakkasid' is used to describe the initiation of the act of 'thinking'.
Sa hakkasid juba mõtlema protesti organiseerimisele.
You started thinking about organizing a protest.
'Hakkasid' here delineates the start of 'learning' new abilities.