Pokaždé když zasáhnu míček, létá přes síť.

It means every time (that) or whenever.

• pokaždé = every time
• když = when

So Pokaždé, když... is a very common Czech pattern for repeated situations:

• Pokaždé, když přijdu domů, jsem unavený. = Every time I come home, I’m tired.

In your sentence, it introduces the idea that the same thing happens again and again.

In standard Czech punctuation, you would normally write:

Pokaždé, když zasáhnu míček, létá přes síť.

That first comma is used because když zasáhnu míček is a subordinate clause.
The second comma marks the end of that subordinate clause.

So the standard punctuation is:

• Pokaždé, když ... , ...

Zasáhnu is the 1st person singular form of zasáhnout.

It means I hit, I strike, or I make contact with.

The ending -nu tells you the subject is I, so Czech does not need já here.

This form is also perfective, which fits well because hitting the ball is seen as a single completed contact, not an ongoing action.

So:

• zasáhnu = I hit / I manage to hit / I make contact with

Because Czech often leaves out subject pronouns when the verb ending already makes the subject clear.

Here, zasáhnu already means I hit, so já is unnecessary.

• Zasáhnu míček = I hit the ball
• Já zasáhnu míček would sound more emphatic, like I’m the one who hits the ball

Czech normally avoids pronouns unless they are needed for contrast or emphasis.

This is a very common aspect question.

• letí = is flying / flies in a more single-event sense
• létá = flies repeatedly / tends to fly / keeps flying

Because the whole sentence describes a repeated situation (every time I hit the ball...), Czech uses létá, the habitual/frequentative form.

So:

• míček letí přes síť = the ball is flying over the net right now / in one specific instance
• míček létá přes síť = the ball goes over the net repeatedly, as a usual result

Because they describe the action in different ways.

• zasáhnu focuses on a single completed moment: I hit the ball
• létá describes the repeated result or regular behavior: it flies over the net

This mixture is very natural in Czech:

• a single completed trigger in the when/whenever clause
• a repeated or habitual result in the main clause

So the sentence means something like:

Whenever I make contact with the ball, it tends to go over the net.

Míček is the direct object of zasáhnu, so it is in the accusative case.

The dictionary form is nominative singular:

• míček = nominative singular

But for many masculine inanimate nouns, the accusative singular looks exactly the same as the nominative:

• nominative: míček
• accusative: míček

So even though the form does not change, the case is still accusative because of its function in the sentence.

Přes means across, over, or through/across to the other side of, and it takes the accusative.

So in:

přes síť = over/across the net

the noun síť is accusative.

The nominative and accusative singular happen to look the same here:

• nominative: síť
• accusative: síť

So again, the case is accusative even though the form does not visibly change.

Míček is a diminutive form of míč.

• míč = ball
• míček = little ball / ball

In sports, míček is very often used naturally for smaller balls, such as a tennis ball, ping-pong ball, etc. It does not always sound especially cute; often it is just the normal word in context.

So:

• míč feels more general or larger
• míček fits well for a small sports ball

Czech often omits subject pronouns when the subject is clear from context.

In létá přes síť, the understood subject is míček from the first clause.

English needs it flies over the net, but Czech can simply say:

• létá přes síť

If you wanted, you could make the subject explicit:

• Pokaždé, když zasáhnu míček, míček létá přes síť.

But that sounds repetitive, so native Czech normally leaves it out.

Not completely. Czech word order is fairly flexible, though the given order is natural and neutral.

Standard neutral version:

Pokaždé, když zasáhnu míček, létá přes síť.

You can move things around for emphasis, for example:

• Pokaždé, když zasáhnu míček, přes síť létá.
  This gives more emphasis to over the net.

But the original order is the most straightforward for a learner.

A repeated habit.

Several things show that:

• pokaždé = every time
• když introduces the repeated situation
• létá is the habitual/frequentative form

So the sentence is not about one specific hit. It describes what usually happens whenever that action occurs.