Por si se me olvida la cita, dejo la libreta encima del sillón y el expediente dentro del bolso.

Por si means in case.

It introduces a precaution: the speaker does something now because a possible problem might happen later.

So:

• Por si se me olvida la cita = In case I forget the appointment

It is not the same as simple si:

• si se me olvida la cita = if I forget the appointment
• por si se me olvida la cita = in case I forget the appointment

The second one explains why the speaker is doing the action in the main clause.

After por si, Spanish normally uses the indicative, not the subjunctive.

So the natural form is:

• por si se me olvida

not:

• por si se me olvide

That is because por si presents something as a real possible situation, not as a wish, doubt, or purpose.

A good rule to remember is:

• por si + indicative

Examples:

• Lleva un paraguas por si llueve.
• Te llamo por si estás en casa.

This is a very common Spanish way to talk about forgetting something, especially when it feels unintentional.

• olvidar = to forget
• olvidarse = to forget / to slip one’s mind

In se me olvida la cita:

• me tells you who is affected: to me
• se is part of the pronominal verb pattern olvidarse
• la cita is the thing that is forgotten

So the idea is roughly:

• the appointment slips my mind
• literally, something like the appointment gets forgotten on me

This often sounds more natural in Spanish than a direct olvido la cita.

Compare:

• Olvido la cita = I forget the appointment
• Se me olvida la cita = The appointment slips my mind / I happen to forget the appointment

Because the grammatical subject is la cita, which is singular.

In se me olvida la cita:

• la cita = subject
• olvida agrees with la cita

That is why it is olvida, not olvidan.

Compare:

• Se me olvida la cita = I forget the appointment
• Se me olvidan las citas = I forget the appointments

This is an important pattern: in this structure, the verb agrees with the thing forgotten, not with the person.

Spanish often uses the present tense for:

• what someone is doing right now
• a habitual action
• a planned or immediate action

So dejo here can mean something like:

• I leave
• I’m leaving
• I leave it there

depending on context.

In this sentence, it sounds like the speaker is describing a practical action they do now as a precaution.

Spanish uses definite articles very often, sometimes more often than English.

Here, the speaker is referring to specific, identifiable things:

• la cita
• la libreta
• el expediente
• el sillón
• el bolso

In English, you might sometimes prefer a possessive like my notebook or my bag, but Spanish does not always need that if the context already makes it clear.

So depending on context, Spanish might say:

• dejo la libreta dentro del bolso

where English might naturally say:

• I leave my notebook inside my bag

Both are normal in their own language systems.

Because del is the contraction of de + el.

So:

• encima de el sillón → encima del sillón
• dentro de el bolso → dentro del bolso

This contraction is mandatory in standard Spanish.

The only common exception is with proper names or titles where el is part of the name, such as:

• de El Escorial

But in ordinary noun phrases like this one, you must use del.

Both can often mean on the armchair or on top of the armchair.

But there is a slight nuance:

• encima de strongly suggests position on top of
• sobre can also mean on, but it is sometimes a bit more neutral or formal

In everyday Spanish, encima del sillón sounds very natural here.

So:

• la libreta está encima del sillón
• la libreta está sobre el sillón

Both are possible, but encima de often feels a little more visual and physical.

They describe two different kinds of location:

• encima de = on top of
• dentro de = inside

So:

• la libreta encima del sillón = the notebook is on the armchair
• el expediente dentro del bolso = the file is inside the bag

This contrast is very common in Spanish:

• encima de la mesa = on the table
• dentro del cajón = inside the drawer

Spanish links them by order.

The sentence groups the elements like this:

• dejo la libreta encima del sillón
• y el expediente dentro del bolso

So:

• encima del sillón goes with la libreta
• dentro del bolso goes with el expediente

This is the normal reading because each location phrase follows the noun it describes.

In Spain, sillón usually means an armchair or large comfortable chair, normally for one person.

• sillón = armchair
• sofá = sofa / couch

So if the sentence says sillón, you should not usually imagine a full sofa unless the context suggests otherwise.

Yes, but it means something slightly different.

• Por si se me olvida la cita = in case I forget the appointment
• Para que no se me olvide la cita = so that I won’t forget the appointment

The difference is:

• por si = precaution against a possible problem
• para que no = purpose or intention

So in your sentence, por si is perfect because the speaker is taking a precaution.

Not usually. In Spain, expediente often means a file, case file, or set of documents kept together.

So it can be more than one sheet of paper. It often refers to an organised folder or official file, especially in administrative, academic, legal, or work contexts.

That is why expediente does not always equal a single documento.