Hễ sách sắp quá hạn là tôi gia hạn.

It is a common Vietnamese pattern meaning whenever ... then ..., every time ..., or sometimes as soon as ....

So the structure is:

• hễ + situation/trigger
• là + result/action

In this sentence:

• hễ sách sắp quá hạn = whenever the book/books are about to become overdue
• là tôi gia hạn = then I renew them / I extend the loan

It usually describes a regular or predictable response.

Yes, là often means to be, but not here.

In hễ ... là ..., là is not the normal copula. It works more like a linker introducing the result, similar to then in English:

• Whenever X happens, then I do Y

So in this sentence, you should not read là as is. It is just part of the pattern hễ ... là ....

It can mean either, depending on context.

Vietnamese nouns often do not show singular/plural unless the speaker wants to make it explicit. So sách can mean:

• a book
• the book
• books
• the books

In this sentence, if the context is library borrowing in general, English often uses books. If the speaker is talking about one borrowed book, book is also possible.

Because classifiers are often omitted when the noun is being used in a general, non-counted way.

Here, sách is just referring to the item(s) in a broad sense, so sách alone is natural.

Compare:

• sách = book/books in general
• cuốn sách này = this book
• một quyển sách = one book

So the lack of a classifier here is normal.

• sắp = about to, soon
• quá hạn = overdue, past the deadline

So sắp quá hạn means about to become overdue or nearly overdue.

It describes a point just before the due date is passed.

It is understandable, but many speakers might choose a slightly different expression in everyday use.

For library items, people often say:

• sắp đến hạn = nearing the due date
• sắp hết hạn = about to expire / about to run out

That is because quá hạn usually describes something that is already late. So sắp quá hạn is understandable as almost overdue, but sắp đến hạn may sound a bit more natural in many situations.

Gia hạn means to extend or to renew a deadline/time period.

In a library context, it means renewing the borrowing period.

It is a very common and useful verb, especially in situations involving:

• library loans
• visas
• contracts
• deadlines
• subscriptions

It sounds a little more formal than some everyday verbs, but it is completely normal here.

Because Vietnamese often leaves out information that is already obvious from context.

Here, the listener can understand that the speaker means something like:

• renew the book loan
• renew the due date
• renew the borrowing period

So tôi gia hạn is enough.

If you want to be more explicit, you could say things like:

• tôi gia hạn sách
• tôi gia hạn thời hạn mượn

But omitting the object is very natural when the meaning is clear.

Yes.

Even without words like thường or hay, the pattern hễ ... là ... strongly suggests a repeated or habitual action.

It gives the idea:

• whenever this situation happens, this is what I do

So the sentence is not about one single event. It sounds like a routine or regular response.

Yes. Some common alternatives are:

• cứ ... là ... = every time ... then ...
• khi ... thì ... = when ... then ...
• nếu ... thì ... = if ... then ...

Each one has a slightly different feel:

• hễ ... là ...: strong trigger-result feeling; whenever X happens, Y follows
• cứ ... là ...: very common in speech; every time X happens, Y happens
• khi ... thì ...: more neutral
• nếu ... thì ...: more conditional, more like if

So hễ is a good choice if you want to emphasize a regular pattern.

Yes, it could be omitted if the context already makes it clear who is acting.

For example, in conversation, Vietnamese often drops subjects that are easy to infer. But keeping tôi here is helpful because it makes the sentence clear and explicit:

• Hễ sách sắp quá hạn là tôi gia hạn.

With tôi, the sentence clearly highlights the speaker’s own habit: Whenever the book/books are nearly overdue, I renew them.