Antes de dormir, dejo la linterna sobre la mesilla por si acaso.

Because after antes de + a verb, Spanish normally uses the infinitive when the subject stays the same.

• Antes de dormir = before sleeping / before going to sleep

You would not say antes de duermo because duermo is a conjugated verb, and that structure does not work after de.

If the subject changes, Spanish often uses antes de que + subjunctive:

• Antes de dormir, dejo la linterna...
  = Before going to sleep, I leave the torch...
  (same subject: I)
• Antes de que llegue mi hermano...
  = Before my brother arrives...
  (different subject)

So here antes de dormir is the normal and correct structure.

Because Antes de dormir is an introductory time phrase placed at the beginning of the sentence.

Spanish often uses a comma after this kind of opening element, especially in clear, careful writing:

• Antes de dormir, dejo la linterna...

You may sometimes see short introductory phrases without a comma in informal writing, but with this sentence the comma is very natural and standard.

Spanish often uses the present tense to describe:

• habits
• routines
• things someone regularly does

So dejo la linterna... means something like:

• I leave the torch...
• I usually leave the torch...
• I put the torch...

Even though English might sometimes prefer I leave or I usually leave, Spanish does not need an extra word like usually if the habitual meaning is clear from context.

In Spanish, definite articles (el, la, los, las) are used more often than in English.

Here, la linterna and la mesilla sound natural because the speaker is referring to specific, known objects in the situation:

• the torch
• the bedside table

Leaving out the articles would sound unnatural in this sentence.

Spanish commonly uses articles with everyday objects even where English might be more flexible.

In Spain, mesilla usually means a small table, and in this context it most naturally means a bedside table / nightstand.

Very common expressions are:

• mesilla
• mesilla de noche

In many contexts in Spain, just mesilla is enough if it is obvious that we are talking about the bedside table.

In other Spanish-speaking countries, people may prefer other terms, such as:

• mesa de noche
• velador
• buró
  depending on the region.

Because sobre means on / on top of, while en usually means in / on / at, depending on context.

Here, sobre la mesilla clearly suggests the torch is on top of the bedside table.

• sobre la mesilla = on the surface of the table

If you said en la mesilla, many speakers might understand it as:

• in the bedside table
• or more loosely at the bedside table, depending on context

So sobre is more precise here.

Yes. Encima de la mesilla is also correct and very natural.

There is a small difference in feel:

• sobre la mesilla = slightly more compact, a bit more neutral or written
• encima de la mesilla = very common in everyday speech, more explicitly on top of

Both are good here.

Por si acaso is a very common fixed expression meaning:

• just in case
• in case
• as a precaution

It often expresses the idea of doing something because something unexpected might happen.

So in this sentence, the idea is:

• I leave the torch on the bedside table just in case.

It is a set phrase, so learners should try to remember it as a chunk.

Because por si acaso often works as a complete fixed expression on its own.

Spanish can also say por si + clause:

• Llévate un paraguas por si llueve.
  = Take an umbrella in case it rains.

But when the rest is obvious or general, Spanish often just says:

• por si acaso

That is why this sentence does not need an extra verb after it.

Not exactly. On its own, acaso can sometimes be related to ideas like perhaps or by chance, depending on the structure, but in por si acaso it is part of a fixed expression.

So it is better not to translate the words one by one here. Learn:

• por si acaso = just in case

as a whole unit.

Yes. Spanish word order is flexible.

These are all possible:

• Antes de dormir, dejo la linterna sobre la mesilla por si acaso.
• Dejo la linterna sobre la mesilla antes de dormir, por si acaso.
• Por si acaso, dejo la linterna sobre la mesilla antes de dormir.

The original version sounds very natural because it starts with the time phrase and ends with the reason/precaution.

So the original order is a good, idiomatic choice.

In this sentence, antes de dormir is best understood as before going to sleep / before sleeping.

Literally, dormir means to sleep, but in time expressions like this, English often prefers before going to sleep because that sounds more natural.

So the Spanish is simple, but the most natural English interpretation may be slightly different.

Yes, very natural.

Everything in the sentence fits everyday Spanish from Spain:

• antes de dormir = common
• dejo for a routine action = common
• linterna = normal word for torch/flashlight
• mesilla = especially natural in Spain
• por si acaso = extremely common

So this is a very natural sentence for Spain.