Pokaždé když hraji čtyřhru, stojím u postranní čáry.

Yes. Pokaždé když literally means every time when, but in natural English it is often best translated as whenever or every time.

• pokaždé = every time
• když = when

Together, they introduce a repeated situation.

In standard written Czech, yes: Pokaždé, když hraji čtyřhru, stojím u postranní čáry.

The part když hraji čtyřhru is a subordinate clause, so it is normally separated by commas. That is why careful written Czech usually has:

• one comma before když
• one comma after čtyřhru

Yes. Both hraji and hraju mean I play.

• hraji is more standard or formal
• hraju is very common in everyday speech

The infinitive is hrát = to play.

Because čtyřhra is the direct object of hraji, and the verb hrát takes the accusative case.

So:

• nominative: čtyřhra
• accusative: čtyřhru

That is why you get hraji čtyřhru = I play doubles.

Čtyřhra is the sports term for doubles—a game with two players on each side, for example in tennis or badminton.

It is formed from the idea of four-person play.

Here it expresses a habitual action, so it means I stand or I am positioned whenever that situation happens.

Czech often uses the present tense for repeated or general actions, just like English does in sentences such as Every time I play doubles, I stand by the sideline.

stojím is the 1st person singular form of stát = to stand.

Because u means by, near, next to, and it requires the genitive case.

So:

• u postranní čáry = by the sideline

This is different from na postranní čáře, which would suggest being on the line rather than simply near it.

It is genitive singular.

The basic form is:

• postranní čára = sideline

After u, it changes to:

• u postranní čáry

The noun čára changes to čáry because u takes the genitive. The adjective postranní agrees with the noun, but its form stays postranní here.

It means the sideline in a sports context.

• postranní = side, lateral
• čára = line

So postranní čára is the line at the side of the court or playing area.

No. Czech word order is fairly flexible.

This version is natural because it begins with the repeated-time expression pokaždé, když... and then gives the main action. But other word orders are also possible, depending on emphasis, for example:

• Když hraji čtyřhru, pokaždé stojím u postranní čáry.
• U postranní čáry stojím pokaždé, když hraji čtyřhru.

The original order sounds neutral and natural.

Because the sentence describes a repeated, general situation, not one completed event.

The verb hrát is imperfective, and that is exactly what Czech normally uses for habits and repeated actions. A perfective verb would not fit well here, because pokaždé refers to something that happens again and again, not to one single completed action.