Rozbité zásuvky jsou pro děti nebezpečné, proto je musíme hned opravit.

Because both adjectives agree with zásuvky.

In Czech, adjectives must match the noun in:

• gender
• number
• case

Zásuvky is feminine plural nominative here, so the adjectives also take the matching plural form: rozbité zásuvky and zásuvky jsou nebezpečné.

For this adjective type, -é is the normal nominative plural ending for feminine nouns.

It is the plural form of zásuvka.

In this sentence, zásuvky is nominative plural, because it is the subject of jsou:

• zásuvka = one socket / outlet
• zásuvky = sockets / outlets

No, not in normal standard Czech.

Czech often leaves out subject pronouns like my, oni, etc., but it normally does not leave out the verb být in a sentence like this.

So:

• Zásuvky jsou nebezpečné = correct
• leaving out jsou would sound wrong here

Pro takes the accusative case, so děti is accusative plural here.

The noun is dítě = child. Its plural is irregular:

• nominative plural: děti
• accusative plural: děti

So the form looks the same in both cases. That is why pro děti means for children.

Because it is also agreeing with zásuvky.

Even though nebezpečné comes after jsou, it still describes the subject zásuvky, so it matches it in gender and number.

So:

• rozbité zásuvky = broken sockets
• zásuvky jsou nebezpečné = the sockets are dangerous

Both adjectives are plural to match zásuvky.

Here proto means therefore, so, or that is why.

It connects the second clause to the first one as a result:

• broken sockets are dangerous for children
• therefore / so we must repair them immediately

It is a very common word for showing consequence.

Because the sentence contains two main clauses:

• Rozbité zásuvky jsou pro děti nebezpečné
• proto je musíme hned opravit

In Czech, when two main clauses are linked like this, a comma is normally used.

Here je does not mean is.

In this sentence, je is a short unstressed pronoun meaning them. It refers back to zásuvky.

So:

• jsou = are
• je = them

This can be confusing because the forms look the same, but their jobs in the sentence are different.

Because short pronouns like je usually go very early in the clause, often in the second position.

So after proto, the clitic pronoun je comes right away:

• proto je musíme hned opravit

This is very natural Czech word order.

If the clause started with the verb, you would also commonly get:

• musíme je hned opravit

So the key idea is: short pronouns tend to stay near the beginning.

Musíme already means we must / we have to.

It is the 1st person plural form of muset:

• musím = I must
• musíš = you must
• musíme = we must

Czech usually does not need the subject pronoun my, because the verb ending already shows who the subject is.

So my musíme is possible, but it is only used when you want extra emphasis.

Because opravit is perfective.

That means it focuses on completing the action: fixing the sockets so that the repair is done.

In this sentence, that is the natural idea:

• we must fix them now, to completion

Opravovat would be imperfective and would sound more like:

• repairing in progress
• repeated repairing
• the activity itself rather than the finished result

After musíme, Czech often uses the perfective verb when the goal is a completed action.

Hned means immediately, right away, or at once.

It adds urgency:

• musíme hned opravit = we must repair them immediately

It is a very common everyday adverb.

Yes, Czech word order is flexible, but not all versions sound equally natural.

The given sentence is very natural:

• Rozbité zásuvky jsou pro děti nebezpečné, proto je musíme hned opravit.

You could also hear:

• Rozbité zásuvky jsou pro děti nebezpečné, proto musíme je hned opravit.

But with short pronouns, Czech usually prefers the pronoun earlier:

• proto je musíme...

So the original version is the best model to learn.

No. Zásuvka can also mean drawer.

The exact meaning depends on context. In this sentence, because they are dangerous for children and need to be repaired, the meaning is clearly electrical sockets / outlets.