דלת הסתרים מובילה למבוי סתום.

A natural pronunciation is:

delet ha-starim movila le-mavoy satum

Roughly for an English speaker:

DEH-let ha-sta-REEM mo-vee-LA le-ma-VOY sa-TOOM

A more fully pointed version would be:

דֶּלֶת הַסְּתָרִים מוֹבִילָה לְמָבוֹי סָתוּם

A few pronunciation notes:

• דלת = delet
• הסתרים = ha-starim
• מובילה = movila
• מבוי = mavoy
• סתום = satum

Because the subject is דלת, and דלת is a feminine singular noun.

In Hebrew, present-tense verb forms agree with the subject in gender and number:

• מוביל = masculine singular
• מובילה = feminine singular
• מובילים = masculine/mixed plural
• מובילות = feminine plural

So:

• דלת ... מובילה = The door ... leads
• not מוביל, because that would be masculine singular

Also, even though הסתרים is plural in form, the whole phrase is still grammatically headed by דלת, so the verb agrees with דלת, not with הסתרים.

Yes. מובילה is the feminine singular present-tense form of להוביל, which means to lead.

So:

• להוביל = to lead
• מוביל = leads / is leading (masculine singular)
• מובילה = leads / is leading (feminine singular)

A useful thing to know is that Hebrew present tense often covers both:

• simple present: leads
• present progressive: is leading

So מובילה can mean either, depending on context.

Because דלת הסתרים is a construct chain (in Hebrew grammar: סמיכות).

In a construct chain, the first noun is linked to the second noun, and definiteness is usually marked on the second noun, not the first.

So:

• דלת סתרים = a secret door / a hidden door
• דלת הסתרים = the secret door

This is similar to other common Hebrew expressions like:

• בית ספר = a school
• בית הספר = the school

So the ה is on הסתרים, but the whole phrase דלת הסתרים becomes definite.

Yes, literally the structure is something like door of the secrets / hidden things. But idiomatically, it means secret door or hidden door.

The noun סתרים is plural in form, and in this kind of expression it belongs to a somewhat literary or fixed phrase. Hebrew often has expressions whose literal structure is not the most natural way to translate them into English.

So:

• literal structure: door of secrets / door of hidden things
• natural English meaning: secret door / hidden door

This is very common in Hebrew. You translate the meaning of the whole phrase, not always each word separately.

Because the verb להוביל usually goes with ל־, meaning to.

So:

• להוביל ל־ = to lead to

That makes:

• מובילה למבוי סתום = leads to a dead end

You may also see אל in some contexts, but להוביל ל־ is the most normal everyday pattern here.

מבוי סתום is a noun + adjective phrase:

• מבוי = alley, passage, lane
• סתום = closed, blocked, sealed

Together, מבוי סתום means dead end or dead-end alley.

Why סתום?

Because מבוי is masculine singular, and Hebrew adjectives must agree with the noun they describe.

So:

• מבוי סתום = masculine singular
• If the noun were feminine singular, the adjective would change too:
  • דרך סתומה = a blocked road

Because the phrase is indefinite here:

• מבוי סתום = a dead end
• המבוי הסתום = the dead end

In Hebrew, when a noun phrase is definite, the adjective usually becomes definite too:

• מבוי סתום = an/a dead end
• המבוי הסתום = the dead end

After the preposition ל־, unpointed Hebrew spelling can hide some of this on the noun itself, but the adjective still helps show it:

• למבוי סתום = to a dead end
• למבוי הסתום = to the dead end

So the lack of ה־ on סתום tells you the phrase is indefinite.

Yes. דלת סודית is perfectly possible, and it sounds more straightforward and everyday.

There are slight flavor differences:

• דלת סודית = secret door
• דלת נסתרת = hidden/concealed door
• דלת הסתרים = a more literary, dramatic, or fixed-expression way to say secret/hidden door

So if you are learning ordinary modern Hebrew, דלת סודית may feel easier.
But דלת הסתרים is a nice expression and often sounds more atmospheric, like something from fantasy, mystery, or formal writing.

It can be both.

Literal:

• a real dead-end alley or passage

Figurative:

• a dead end in a discussion
• an impasse in negotiations
• a situation with no way forward

So Hebrew speakers can say that a plan, argument, or political process has reached a מבוי סתום, just like English dead end or impasse.