Antes de salir de casa, miré el índice otra vez y metí el subrayador en la mochila por si hacía falta.

In Spanish, casa often works like home in English when you mean your home in a general sense.

So:

• salir de casa = to leave home
• estar en casa = to be at home
• volver a casa = to return home

If you say la casa, it sounds more like a specific physical house/building:

• salir de la casa = leave the house

Both are possible, but de casa is the natural choice when the idea is simply home, not the building itself.

They are in the preterite because they describe completed actions in a story:

• miré = I looked
• metí = I put / I stuck

The sentence is narrating a sequence of actions:

1. miré el índice otra vez
2. metí el subrayador en la mochila

These are single, finished events, so the preterite fits well.

If the sentence were describing background or an ongoing situation, Spanish would be more likely to use the imperfect instead.

Índice usually means index or table of contents, depending on context.

Common meanings:

• el índice de un libro = the index
• el índice / el índice de contenidos = the table of contents
• el dedo índice = the index finger

In this sentence, it most likely refers to something in a book, notebook, or document that the speaker checked again before leaving.

Yes. Otra vez and de nuevo can both mean again here.

• miré el índice otra vez
• miré el índice de nuevo

Both are natural.

In everyday speech, otra vez is very common.
Sometimes otra vez can also sound a bit like yet again, depending on tone, but in this sentence it simply means again / one more time.

Subrayador is a highlighter or a marker used for underlining/highlighting text.

It comes from the verb subrayar = to underline / to highlight.

In Spain, learners may also hear:

• un subrayador
• un rotulador fluorescente
• sometimes other regional terms depending on the place

So here metí el subrayador en la mochila means the speaker put their highlighter into their backpack.

With meter, Spanish normally uses en because the idea is putting something inside something else.

• meter algo en la mochila = put something in the backpack
• meter las llaves en el bolsillo = put the keys in your pocket

Even though there is movement, Spanish uses en after meter because the focus is the final location: inside.

Using a la mochila would not sound natural here.

Both can sometimes be translated as to put, but they are not always interchangeable.

• poner = put/place
• meter = put/stick/place inside

Because the object is going into the backpack, meter is especially natural.

Compare:

• Puse el libro en la mesa = I put the book on the table
• Metí el libro en la mochila = I put the book in the backpack

So meter emphasizes putting something inside a container or enclosed space.

It means in case it was needed or more naturally in English, in case I needed it.

So the idea is:

• The speaker put the highlighter in the backpack
• because it might be needed later

Spanish often leaves the subject of hacía falta unstated, so English may need to make it more explicit:

• por si hacía falta = in case it was needed
• por si me hacía falta = in case I needed it

In this sentence, the meaning is easy to understand from context even without me.

Hacía falta is imperfect because it refers to a possible need, not a specific completed event.

The speaker is thinking:

• I took the highlighter along in case it might turn out to be necessary.

That is not a finished action like it ended up being necessary once. It is more like a background possibility.

So:

• por si hacía falta = in case it was needed / might be needed

If you said hizo falta, it would suggest that it actually became necessary as a completed event, which is not the intended meaning here.

Because after por si, Spanish normally uses the indicative, not the subjunctive.

This is a very common learner question because English in case can feel hypothetical, but Spanish behaves differently here.

Natural patterns are:

• Llévatelo por si hace falta. = Take it in case it’s needed.
• Me lo llevé por si hacía falta. = I took it in case it was needed.

So in this sentence, hacía falta is correct and natural.

Because Spanish often omits subject pronouns when the verb form already shows who the subject is.

• miré clearly means I looked
• metí clearly means I put

So yo is unnecessary unless the speaker wants emphasis or contrast:

• Yo miré el índice, no ella. = I looked at the index, not her.

In neutral narration, leaving out yo is the normal choice.

Because after antes de, Spanish uses the infinitive when the subject is the same as in the main clause.

Here, the same person does both actions:

• salir de casa
• miré
• metí

So Spanish says:

• Antes de salir de casa... = Before leaving home...

If the subject changed, Spanish would usually use antes de que + subjunctive:

• Antes de que salieras de casa... = Before you left home...

Yes, it could, but it would sound a bit more explicit than necessary.

• de casa = from home
• de mi casa = from my house / from my home

In many contexts, Spanish prefers the simpler de casa when the meaning is obviously home. Adding mi is possible, but less idiomatic here unless you want emphasis or contrast.

Usually yes. Mochila normally means backpack or rucksack.

So:

• meter el subrayador en la mochila = put the highlighter in the backpack

Depending on context, it could be a school backpack, hiking backpack, or similar bag worn on the back.