Nie mogę wejść do domu, bo zapomniałem kluczy.

Polish usually drops subject pronouns when they are not needed.

In Nie mogę wejść do domu, bo zapomniałem kluczy, the forms mogę and zapomniałem already show that the subject is I.

So:

• mogę = I can
• zapomniałem = I forgot

You could say Ja nie mogę wejść do domu..., but ja is usually added only for emphasis, contrast, or emotion.

For example:

• Ja nie mogę wejść do domu, ale ty możesz.
  I can’t enter the house, but you can.

Nie mogę means I can’t or I’m not able to.

It comes from the verb móc = to be able to / can.

Here is the present-tense singular:

• mogę = I can
• możesz = you can
• może = he/she/it can

With negation:

• nie mogę = I can’t

Notice that in Polish, nie is written as a separate word before the verb.

This is a question of aspect, which is very important in Polish.

• wejść = to enter / to go in once, as a completed action
• wchodzić = to be entering / to go in repeatedly / habitually

In this sentence, the speaker means a single completed action:
I can’t enter the house.

So wejść is the natural choice.

Compare:

• Nie mogę wejść do domu. = I can’t get into the house.
• Nie mogę wchodzić do domu. = I’m not allowed / not able to be going into the house (more general, repeated, or ongoing).

Because after verbs of movement like wejść when you mean movement into a place, Polish often uses do + genitive.

So:

• do domu = into the house / to the house

The noun dom changes form after do:

• dom = house/home
• do domu = to/into the house/home

This is because do requires the genitive case.

Because do takes the genitive case, and dom in the genitive singular is domu.

So:

• nominative: dom
• genitive: domu

This is very common in Polish. Prepositions often force a specific case, and the noun changes form accordingly.

Examples:

• do sklepu = to the shop
• do szkoły = to school
• do domu = home / into the house

Bo means because.

Yes, you can often use ponieważ instead, but there is a difference in style:

• bo = very common, everyday, conversational
• ponieważ = more formal or more written

So:

• Nie mogę wejść do domu, bo zapomniałem kluczy.
  natural everyday Polish

• Nie mogę wejść do domu, ponieważ zapomniałem kluczy.
  also correct, but a bit more formal

Zapomniałem means I forgot and tells us several things at once:

• z- / zapomnieć = the verb to forget in perfective form
• -łem = first person singular past masculine

So zapomniałem means:

• I forgot — said by a male speaker

This is one reason Polish often does not need the pronoun ja: the verb ending already gives a lot of information.

A woman would say:

Nie mogę wejść do domu, bo zapomniałam kluczy.

The only change is:

• zapomniałem → zapomniałam

This is because Polish past tense agrees with the speaker’s gender.

So:

• zapomniałem = I forgot (male speaker)
• zapomniałam = I forgot (female speaker)

Because after zapomnieć, Polish traditionally uses the genitive case.

So:

• klucze = keys (nominative/accusative plural)
• kluczy = of keys / keys in the genitive plural

That is why:

• zapomniałem kluczy = I forgot the keys

This is something learners often just have to memorize with the verb:

• zapomnieć czegoś = to forget something

You may sometimes hear accusative forms in casual speech, but genitive is the standard form to learn here.

Not in normal English, but that is a useful way to notice the grammar.

Polish and English do not always structure objects the same way. In English we say:

• I forgot the keys

In Polish, the verb zapomnieć commonly takes the genitive, so the noun appears in a form that does not match the English direct object pattern.

So it is best to learn it as a Polish verb pattern:

• zapomnieć + genitive

For example:

• zapomniałem telefonu = I forgot my phone
• zapomniałem nazwiska = I forgot the surname/name

In this sentence, do domu can often be understood as home in English, even though the literal meaning is to/into the house.

That is why the sentence could be translated naturally as either:

• I can’t get into the house, because I forgot the keys.
• I can’t get into the house / home, because I forgot my keys.

Polish dom can mean:

• house
• home

The exact English choice depends on context.

Yes. Polish word order is more flexible than English word order, because cases and verb endings show grammatical relationships.

The sentence:

Nie mogę wejść do domu, bo zapomniałem kluczy.

is the most neutral, natural order.

But you could also say things like:

• Bo zapomniałem kluczy, nie mogę wejść do domu.
• Nie mogę do domu wejść, bo zapomniałem kluczy.

These alternatives are grammatically possible, but they may sound more marked, emphatic, or less neutral.

For learners, the original version is the best one to use.

A rough pronunciation guide is:

Nyeh MOH-geh VAYSHCH doh DOH-moo, boh zah-pohm-NYAH-wem KLOO-chih

A few tricky parts:

• Nie sounds roughly like nyeh
• mogę has ę, which is a nasal vowel; at the end of a word it is often pronounced approximately like e with some nasal coloring
• wejść is tricky: it sounds roughly like vayshch
• ł in zapomniałem sounds like English w
• kluczy sounds roughly like KLOO-chih

This is only approximate, but it can help at the beginning.