Dejo la mochila detrás de la puerta cuando llego a casa.

In this sentence, the present tense is being used for a habit or routine: something the speaker usually does.

So dejo means I leave / I put / I place, depending on context.

Here it is not necessarily describing what is happening right now at this exact second. It can mean:

• This is what I do when I get home
• I usually leave my backpack behind the door

Spanish uses the simple present a lot for regular actions, just like English does.

Spanish often drops subject pronouns when they are not needed, because the verb ending already shows who the subject is.

• dejo = I leave
• llego = I arrive / I get

So yo dejo is possible, but yo is unnecessary unless you want emphasis or contrast, for example:

• Yo dejo la mochila, pero mi hermano la tira en el sofá.

In a neutral sentence, leaving out yo sounds more natural.

Spanish often uses the definite article (el, la, los, las) where English would prefer a possessive like my.

So la mochila can still clearly mean my backpack, especially when it is obvious from the context that it belongs to the speaker.

This is very normal in Spanish. Compare:

• Me lavo las manos = literally I wash the hands, but naturally I wash my hands
• Dejo la mochila = I leave my backpack

If you say mi mochila, it is still correct, but it sounds more explicit or more contrastive.

Because detrás normally needs the preposition de before the thing it is behind.

So the structure is:

• detrás de + noun

Examples:

• detrás de la puerta
• detrás del coche
• detrás de mí

This is fixed grammar. You cannot normally say detrás la puerta.

Because detrás requires de, and then la puerta is the noun phrase that follows.

So it breaks down like this:

• detrás = behind
• de = of
• la puerta = the door

Together: behind the door

This is just how that expression works in Spanish.

Because the sentence describes a usual action, not one specific event in the past or future.

• cuando llego a casa = when I get home / whenever I get home
• cuando llegué a casa = when I got home (one past event)
• cuando llegaré a casa is not natural here

In this sentence, both verbs are in the present because the whole sentence is talking about a repeated routine:

• I leave the backpack behind the door when I get home

In Spanish, casa often appears without an article when it means home rather than a specific physical house.

So:

• llego a casa = I get home
• estoy en casa = I am at home
• vuelvo a casa = I return home

But if you are talking about a specific house as a building, then la casa is possible:

• Llego a la casa de mi abuela = I arrive at my grandmother’s house

Here, a casa means home, so no article is used.

Because the verb llegar normally uses the preposition a to express a destination.

• llegar a casa
• llegar al trabajo
• llegar a Madrid

English learners sometimes want to use en because English says in or at in some contexts, but with llegar, Spanish uses a.

So:

• correct: llego a casa
• incorrect here: llego en casa

Yes. You can also say:

• Cuando llego a casa, dejo la mochila detrás de la puerta.

This means the same thing.

The original version starts with the main action:

• Dejo la mochila...

The alternative starts with the time clause:

• Cuando llego a casa...

Both are natural. The choice depends on what you want to emphasise first.

Because cuando can be followed by the present tense when talking about:

• habits
• repeated actions
• things that generally happen

So:

• Cuando llego a casa, dejo la mochila... = habitual

In other contexts, Spanish may use a different tense after cuando, especially when referring to the future:

• Cuando llegue a casa, te llamo. = When I get home, I’ll call you.

That is a different structure. In your sentence, it is a routine, so the present tense is correct.

It can feel like either in English, depending on how you translate it.

Dejar often means:

• to leave
• to put
• to set down

In this sentence, it means something like:

• I leave my backpack behind the door
• I put my backpack behind the door

So the exact English word may vary, but the Spanish is completely natural.

Yes, in some contexts, but the nuance changes a little.

• Dejo la mochila detrás de la puerta = I leave it there / that is where I place it
• Pongo la mochila detrás de la puerta = I put it there

Poner focuses more directly on the act of placing something. Dejar often sounds very natural for leaving an object somewhere.

In this sentence, dejo is a very good choice.

Because the written accent shows the stress: de-TRÁS.

Without getting too technical, the accent mark helps show the correct pronunciation and distinguishes the standard written form.

So you should write:

• detrás

not:

• detras

Yes. In context, it can express a regular situation, so it may feel like either:

• when I get home
• whenever I get home

English sometimes makes that distinction more clearly than Spanish does, but in Spanish the present tense with cuando often naturally suggests a repeated action.

Yes. It is completely natural in Spain.

Nothing in the sentence is especially regional or unusual. A speaker in Spain would understand and use it normally.

The only small note is that vocabulary can vary by country:

• mochila is widely understood everywhere
• in some contexts, different countries might prefer different everyday expressions for similar objects or routines

But this sentence works perfectly well in Spain Spanish.