Na té stránce musím napsat heslo, protože bez něj e-mail neotevřu.

Here it most naturally means on that page in the sense of a web page / site page / screen page.

Czech stránka can mean:

• a page in a book
• a web page
• sometimes even a whole website, depending on context

With websites and online pages, Czech very often uses na:

• na stránce = on the page
• na webu = on the web/site

So Na té stránce musím napsat heslo means something like On that page, I have to enter a password.

Because na here expresses location (on that page), and after na in this meaning Czech uses the locative case.

The basic forms are:

• ta stránka = that page
• na té stránce = on that page

Both words change:

• ta → té
• stránka → stránce

So this is a normal case change:

• nominative: ta stránka
• locative: na té stránce

Musím is the 1st person singular form of muset, which means must / have to.

So:

• musím = I must / I have to
• musíš = you must
• musí = he/she/it must

In this sentence:

• musím napsat heslo = I have to write/type/enter the password

Czech often uses muset where English uses either must or have to.

Because napsat is perfective, while psát is imperfective.

• psát = to write, to be writing, to write in general
• napsat = to write down / write out / complete the writing of something

After musím, Czech often uses the perfective infinitive when talking about one complete action:

• musím napsat heslo = I need to enter/write the password completely

If you said musím psát heslo, it would sound more like:

• I have to be writing the password
• I have to spend time writing it

That is less natural here. In a login context, the idea is a single completed action, so napsat fits better.

Yes. In Czech, napsat can be used even when you are typing something, not only writing it by hand.

So in computer/internet contexts:

• napsat heslo often means to type in / enter a password

A very natural English translation here is:

• I have to enter the password or
• I have to type in the password

So don't understand napsat too literally as only pen-and-paper writing.

Yes. Heslo has several meanings, but here it clearly means password.

Possible meanings of heslo include:

• password
• motto
• slogan
• watchword

In a sentence with a web page and e-mail, heslo definitely means password.

Very common Czech computer vocabulary:

• uživatelské jméno = username
• heslo = password

Něj refers back to heslo.

So:

• heslo = password
• bez něj = without it

This is basically Czech for without it, where it means the password.

Because the preposition bez (without) requires the genitive case, and with Czech 3rd-person pronouns, forms after prepositions usually begin with n-.

So:

• on / ono = he / it
• after a preposition: něj / něho

Examples:

• bez něj = without him/it
• pro něj = for him/it
• na něj = onto him/it / for it, depending on context
• k němu = to him/it

The short form ho is not used after prepositions like this.

Also, since heslo is neuter, English would say it, but Czech uses the same oblique pronoun form něj / něho here.

Yes, absolutely.

Both are correct:

• bez hesla e-mail neotevřu
• bez něj e-mail neotevřu

The version with bez něj avoids repeating heslo and sounds natural.

So the difference is mostly stylistic:

• bez hesla = without the password
• bez něj = without it

It does look like a present-tense form, but otevřít is a perfective verb. In Czech, perfective verbs do not normally have a true present meaning. Their present forms usually express the future.

So:

• otevřít = to open (perfective, one completed action)
• otevřu = I will open
• neotevřu = I will not open / I won’t open

That is why:

• bez něj e-mail neotevřu = without it, I won’t open the e-mail

This is a very important Czech pattern:

• perfective present form → future meaning

Because neotevřu is better for a single completed action: opening the e-mail.

Compare:

• neotevřu = I won’t open it
• nebudu otevírat = I won’t be opening it / I won’t be in the process of opening it

Here the speaker means that the action simply will not happen successfully unless they have the password. So perfective neotevřu is the natural choice.

Yes, Czech word order is flexible.

This sentence could also be said as:

• protože bez něj neotevřu e-mail

Both are grammatical.

The version in your sentence:

• bez něj e-mail neotevřu

puts a little more focus on e-mail before the verb.

Czech often moves words around for emphasis, information structure, and style. English word order is much more fixed, but Czech is freer as long as the case endings and verb forms make the relationships clear.

Because Czech has no articles.

English distinguishes:

• a password
• the password

Czech simply says:

• heslo

And the exact meaning depends on context.

So:

• napsat heslo can mean write a password, write the password, or enter the password
• e-mail neotevřu can mean I won’t open an e-mail or I won’t open the e-mail

In this sentence, context makes it clear that a specific password and a specific e-mail are meant.

Yes. Protože is the standard everyday word for because.

So:

• ..., protože bez něj e-mail neotevřu. = ..., because without it I won’t open the e-mail.

It introduces a subordinate clause, but Czech does not force a very rigid word order after it in the way some learners expect. That is why the clause can still look fairly flexible inside:

• protože bez něj e-mail neotevřu
• protože bez něj neotevřu e-mail

Both are fine.