Když zapomenu heslo, moje sestra mi ho vždycky najde v telefonu.

Když means when in the sense of whenever or if/when it happens that.

In this sentence, Když zapomenu heslo... means something like:

• When I forget the password...
• Whenever I forget the password...

So it introduces the situation under which the second part happens.

A useful contrast:

• když = when / whenever
• jestli / pokud = if in a more conditional sense

In everyday Czech, když is very common in sentences like this.

Because zapomenu is from the perfective verb zapomenout = to forget as a completed event.

Czech often uses:

• perfective verbs for single completed actions
• imperfective verbs for repeated, ongoing, or habitual actions

Here, the meaning is when I forget the password as a complete event, so zapomenu fits well.

Compare:

• Když zapomenu heslo... = When I forget the password...
• Když zapomínám hesla... = When I tend to forget passwords / when I am forgetting passwords...
  This sounds more habitual or process-like and would not fit this sentence as naturally.

This is a very common Czech pattern.

With perfective verbs, forms that look like the present tense usually have future meaning.

So:

• zapomenu literally looks like I forget
• but with a perfective verb it really means I will forget / whenever I forget

That is why Když zapomenu heslo... is natural Czech.

The same thing happens in the main clause:

• najde = from perfective najít
• it means she will find / she finds (as a completed result)

So the whole sentence uses perfective verbs to describe repeated completed events: whenever I forget it, she finds it.

Because heslo is the direct object of zapomenu.

The verb zapomenout usually takes the accusative case for the thing forgotten.

• nominative: heslo
• accusative: heslo

For this neuter noun, nominative and accusative singular are the same, so the form stays heslo.

So:

• zapomenu heslo = I forget the password

These are two short pronouns:

• mi = to me / for me
• ho = it / him

In this sentence:

• mi is the indirect object = for me
• ho refers back to heslo = the password

So:

• moje sestra mi ho vždycky najde
  = my sister always finds it for me

Because heslo is grammatically neuter, learners often expect a neuter pronoun like to. But after many verbs, the short unstressed pronoun used for it is ho here.

Czech short unstressed words, often called clitics, tend to appear in a special position in the sentence, and they also have a usual internal order.

In a sequence like this, mi normally comes before ho:

• sestra mi ho najde

That is the most natural standard order here.

A good thing to learn as a pattern is:

• mi ti mu jí nám vám jim often come before
• ho ji je se and similar short pronouns

So mi ho is the expected order.

Yes — and that is exactly the point.

Czech distinguishes strongly between:

• hledat = to look for / search for
• najít = to find

Here the sentence says the sister successfully locates the password, so Czech uses najde from najít.

Compare:

• Sestra hledá heslo. = My sister is looking for the password.
• Sestra najde heslo. = My sister finds the password.

So najde v telefonu means she actually manages to find it in the phone.

Because v means in, and after v for location, Czech uses the locative case.

The noun telefon changes like this:

• nominative: telefon
• locative: telefonu

So:

• v telefonu = in the phone

This means the password is found inside/on the phone, for example in saved passwords, notes, messages, or settings.

Vždycky means always.

It tells us that this is not just a one-time event but something that happens regularly:

• Když zapomenu heslo, moje sestra mi ho najde v telefonu.
• Whenever I forget the password, my sister always finds it on the phone.

Without vždycky, the sentence would still make sense, but it would sound less clearly habitual.

Both are possible, but moje sestra is more explicit: my sister.

Czech often omits possessives when ownership is obvious from context, but keeping moje is completely normal and often clearer, especially for learners.

So:

• moje sestra = my sister
• sestra = sister / the sister, depending on context

In this sentence, moje sestra makes the relationship clear right away.

Yes. Czech word order is flexible, but different orders change emphasis.

The neutral version here is:

• Když zapomenu heslo, moje sestra mi ho vždycky najde v telefonu.

You could also hear things like:

• Když zapomenu heslo, moje sestra ho mi vždycky najde v telefonu.
  This is not the normal order of the short pronouns, so it sounds wrong.

But these are possible:

• Když zapomenu heslo, moje sestra mi ho v telefonu vždycky najde.
• Když zapomenu heslo, vždycky mi ho moje sestra najde v telefonu.

These variants are grammatical, but the original sentence is a very natural neutral order.

In practice, yes, especially in everyday English translation.

Když zapomenu heslo... can be understood as:

• When I forget the password...
• If I forget the password...

But the Czech feeling is often closer to whenever that happens rather than a purely hypothetical if.

So if you want to understand the sentence naturally, think:

• Whenever I forget the password, my sister always finds it for me in the phone.

Yes.

Czech usually does not need subject pronouns like já unless they are emphasized.

The verb ending already tells you the person:

• zapomenu = I forget / I will forget

So já is unnecessary here.

If you said:

• Když já zapomenu heslo...

that would add emphasis, something like:

• When I am the one who forgets the password...

But in a normal sentence, Czech simply says Když zapomenu heslo...