Sabihin po ninyo kung kailan bukas ang silid-aklatan.

Sabihin comes from the root sabi, meaning say or tell. In this sentence, sabihin is being used as a polite command, so it means something like tell or please tell.

A useful thing to know is that this form is very common when asking someone to state or give some information.

Po is a politeness particle. It does not have a direct one-word English translation, but it adds respect.

So:

• Sabihin ninyo = Tell
• Sabihin po ninyo = Please tell / Tell, respectfully

In Filipino, po is often placed near the beginning of the sentence, usually after the first word or phrase.

This is a very common learner question.

Ninyo is used because sabihin is the kind of verb that takes a pronoun from the genitive set, not the ang set.

Compare:

• kayo = the ang form for you plural or you polite
• ninyo = the genitive form for you plural or you polite

So with sabihin, you say sabihin ninyo, not sabihin kayo.

A rough comparison:

• Magsabi kayo = You tell/speak
• Sabihin ninyo = Tell it / say it

For a learner, the important thing is: after sabihin, ninyo is the correct form.

It can do both.

Ninyo can mean:

• you all
• you, when speaking politely to one person

So Sabihin po ninyo... may be addressed either to several people or to one person respectfully.

This is similar to how Filipino often uses plural forms for politeness.

Kailan by itself means when.

But in a sentence like this, kung introduces the embedded clause. So kung kailan means something like when in the sense of as to when or about when.

Compare:

• Kailan bukas ang silid-aklatan? = When is the library open?
• Sabihin po ninyo kung kailan bukas ang silid-aklatan. = Please tell me when the library is open.

So kung helps connect the main command to the information being asked for.

Here, bukas means open.

That is because the context makes it clear:

• kung kailan bukas ang silid-aklatan = when the library is open

Even though bukas can also mean tomorrow, that meaning would not fit naturally here.

So this sentence is about the library being open, not about tomorrow.

Silid-aklatan is the Filipino word for library.

It is a compound word:

• silid = room
• aklat = book
• -an often helps form a place-related noun

So the word has the sense of a place or room for books, which became the standard word for library.

In everyday speech, many speakers also simply say library, but silid-aklatan is the standard Filipino term.

It is hyphenated because it is a compound form. The hyphen helps show that the parts belong together as one word.

For learners, the main thing is simply to recognize silid-aklatan as a single vocabulary item meaning library.

Ang marks the noun phrase that is being treated as the topic or main noun of the clause.

In bukas ang silid-aklatan, the structure is literally closer to:

• Open the library

But the natural English meaning is:

• The library is open

So ang silid-aklatan is the part meaning the library.

Filipino often uses predicate-first word order.

So instead of saying:

• The library is open

Filipino commonly says:

• Bukas ang silid-aklatan
• literally, Open the library

This is normal Filipino structure. The descriptive word bukas comes first, and the ang-marked noun follows.

A more formal or bookish alternative would be:

• Ang silid-aklatan ay bukas

But Bukas ang silid-aklatan is very natural.

It is not stated explicitly in this sentence.

Filipino often leaves out things that are understood from context. In a request like this, the person being told is obvious, so sa akin does not have to be said.

If you want to make it explicit, you could say:

• Sabihin po ninyo sa akin kung kailan bukas ang silid-aklatan.

That means Please tell me when the library is open.

Yes, it is polite and somewhat formal because of po and ninyo.

It sounds appropriate when speaking respectfully to:

• a stranger
• an older person
• an employee or staff member
• several people

A less formal version to one familiar person would be:

• Sabihin mo kung kailan bukas ang silid-aklatan.

A very natural polite request could also be:

• Pakisabi po kung kailan bukas ang silid-aklatan.

So the original sentence is correct and polite, especially in formal situations.