Learn Croatian: Vježbamo u teretani tri puta tjedno. - We exercise in the gym three times a week.

Tjedno is actually an adverb of frequency that translates to 'weekly'. In Croatian, it is much more natural to say 'three times weekly' (tri puta tjedno) than to use the noun for week. You can also do this with other time periods, like dnevno (daily) or mjesečno (monthly).

Why is the word for 'times' spelled puta here instead of put?

The base word for 'time' (in the sense of occurrences) is put. In Croatian, the numbers 2, 3, and 4 must be followed by a specific case called the genitive singular. The genitive singular form of put is puta. So for 'three times', we say tri puta.

Why does the word for gym end with an 'i' (u teretani) instead of a 'u' (u teretanu)?

This is a great reminder of how location cases work! Because we are already inside the gym doing the action (exercising), we use the locative case, changing the 'a' in teretana to an 'i'. If you were talking about going to the gym (a movement or destination), you would use the accusative case u teretanu.

Does the frequency phrase tri puta tjedno have to go at the end of the sentence?

No, Croatian word order is quite flexible. You could also say Tri puta tjedno vježbamo u teretani. Moving the frequency adverbs to the front of the sentence just places a little more emphasis on how often you work out, rather than the fact that you do it in the gym.