Czech does not use possessive pronouns the same way as english. The verb already shows the ownership. When we say for example Máš v kapse klíč. (You have a key in your pocket) - MÁŠ - shows that we speak about you. V KAPSE -the pocket is on your clothes so it can be only yours and there is no need to emphasize it with a possesive pronoun. KLÍČ- when there is no possesive pronoun then the key belongs to you. When there is a possesive pronoun then the key belongs to that person according to the pronoun.
Neither of the following "translations" is idiomatic English:
1. You have a fly in a beer.
2. You have a fly in the beer.
On the other hand, you do not always need the personal possessive pronoun in English. All the following are correct,with different meanings:
1. You have a beer in the fridge.
2. You have a beer in a fridge.
3. You have a beer in your fridge.