Si pasas por la librería, compra una novela fácil.

This is a very common false friend.

• librería = bookstore / bookshop
• biblioteca = library

So in this sentence, la librería is the place where you buy books, not the place where you borrow them.

After si meaning if, Spanish normally uses the present indicative for real or likely future conditions:

• Si pasas por la librería... = If you stop by / go by the bookstore...

Even though the meaning is future in English, Spanish still uses the present here.

So:

• Si pasas... = correct
• Si pasarás... = incorrect here; that form is not used for this kind of if clause
• Si pases... = also incorrect here; Spanish does not usually use the subjunctive after si in this type of sentence

A very common pattern is:

• Si + present indicative, imperative
• Si tienes tiempo, llámame.
• Si ves a Ana, dile hola.

Spanish often leaves out subject pronouns when they are already clear from the verb form.

• pasas already tells you the subject is tú
• compra here is also a tú command

So Si pasas por la librería... is completely natural.
You could say Si tú pasas por la librería..., but that usually adds emphasis or contrast.

Because the second part of the sentence is giving an instruction.

• compra = buy (affirmative tú command)
• compras = you buy (present indicative)

So:

• Si pasas por la librería, compra una novela fácil.
  = If you stop by the bookstore, buy an easy novel.

But:

• Si pasas por la librería, compras una novela fácil.
  sounds like a statement about what you do, not an instruction.

For -ar verbs, the affirmative tú command looks like the él/ella/usted present form:

• comprar → compra
• hablar → habla
• mirar → mira

Because pasar por is the natural expression for to go by, to pass by, or sometimes to stop by a place.

• pasar por la librería = go by / stop by the bookstore

If you said ir a la librería, that would simply mean to go to the bookstore.

So the difference is roughly:

• pasar por la librería = go by it / drop in while passing
• ir a la librería = go there as your destination

It can suggest either, depending on context.

In this sentence, it most naturally means something like:

• if you happen to go by the bookstore
• if you stop by the bookstore

Because the second clause tells you to buy something there, English might naturally translate it as stop by. But literally, pasar por is closer to pass by / go by.

Because si without an accent means if.

Compare:

• si = if
• sí = yes

So in this sentence:

• Si pasas por la librería... = If you stop by the bookstore...

Not:

• Sí pasas... = that would not fit here

Because Spanish usually uses an article with a singular countable noun when you mean one unspecified item.

• una novela fácil = an easy novel

Without the article, novela fácil would not sound natural here as a normal object after compra.

So una is needed because the speaker is asking for one easy novel, without specifying which one.

In Spanish, adjectives often come after the noun.

• una novela fácil = an easy novel

That is the normal order here.

If you moved the adjective before the noun—una fácil novela—it would sound unusual and more literary or stylistically marked. In everyday Spanish, una novela fácil is the natural choice.

It means an easy novel, usually in the sense of:

• easy to read
• not too difficult
• simple in language or style

It does not usually mean morally easy or something strange like that. The context of buying a book makes the meaning clear.

Depending on context, a Spanish speaker might also say:

• una novela sencilla
• una novela fácil de leer

But una novela fácil is perfectly understandable.

Because the sentence starts with an if-clause:

• Si pasas por la librería, compra una novela fácil.

In Spanish, when a subordinate clause like this comes first, it is normally separated by a comma from the main clause.

If the order were reversed, the comma is often omitted:

• Compra una novela fácil si pasas por la librería.

So the comma here is standard punctuation.

Yes. This sentence uses the tú form, so it is informal and singular:

• pasas = you (informal singular)
• compra = command to you (informal singular)

If you wanted to make it formal, you would use usted forms:

• Si pasa por la librería, compre una novela fácil.

That would be more polite or formal.

Yes, depending on context. It can mean:

• a one-time instruction: If you stop by the bookstore, buy an easy novel
• a general instruction for the future: Whenever/if you go by the bookstore, buy an easy novel

The grammar itself does not force only one interpretation. Context tells you whether it is about one occasion or a general plan.