Eu deixo a mochila junto à porta para não a esquecer.

Yes, Eu can be left out.

Portuguese usually does not need the subject pronoun because the verb ending already shows who the subject is. Deixo clearly means I leave / I put / I keep.

So both of these are natural:

• Eu deixo a mochila junto à porta...
• Deixo a mochila junto à porta...

Including Eu can add emphasis, contrast, or clarity.

Deixo is the present indicative of deixar.

Here it can express a habit or a regular action, like something the speaker usually does:

• I leave the backpack by the door so I don’t forget it

In Portuguese, the present tense often covers both:

• a general habit
• something happening now, depending on context

Portuguese uses definite articles more often than English does.

So a mochila literally means the backpack, and this sounds natural when talking about a specific backpack that both speaker and listener can identify from context.

In English, we might sometimes say I leave my backpack by the door, but Portuguese often uses the article where English would use a possessive or no article.

Junto a means next to, by, or close to.

So junto à porta means:

• next to the door
• by the door

It suggests the backpack is placed very near the door, not just somewhere in the same room.

Because à is a contraction of:

• a = the preposition to / at / by
• a = the feminine singular definite article the

So:

• junto a + a porta → junto à porta

The accent shows that these two words have merged.

You will see the same pattern with other feminine singular nouns:

• à escola
• à janela
• à entrada

Para means for or in order to, and here it introduces a purpose.

So para não a esquecer means:

• so as not to forget it
• in order not to forget it

The word não simply negates the verb that follows.

That a is not an article here. It is a direct object pronoun meaning it, referring back to a mochila.

Because mochila is feminine singular, the pronoun is a.

So:

• a mochila → a = it

This avoids repeating the noun:

• para não esquecer a mochila
• para não a esquecer

Both are possible, but the second is less repetitive.

In this sentence, não helps trigger the pronoun to come before the verb.

So you get:

• não a esquecer

This is very natural in European Portuguese. The negative word não attracts the pronoun to a position before the verb it belongs to.

The important point for a learner is:

• a refers to a mochila
• não helps place that pronoun before esquecer

You could say deixo-a, but only if you are replacing a mochila with a pronoun.

Compare:

• Eu deixo a mochila junto à porta...
• Eu deixo-a junto à porta...

Both are correct, but you normally do not say both together:

• not natural: Eu deixo a mochila-a...

So in the original sentence, the noun is stated first, and later the pronoun a refers back to it.

Yes, and that is also natural Portuguese, but it uses a different structure.

There are two common patterns:

• esquecer alguma coisa = to forget something
• esquecer-se de alguma coisa = to forget something

So these are both possible:

• para não a esquecer
• para não me esquecer da mochila

The original sentence uses the simpler direct object structure. The alternative uses the reflexive form esquecer-se de.

Yes. Portuguese allows that.

You can say:

• Para não a esquecer, eu deixo a mochila junto à porta.

This puts more focus on the purpose first.

The original order is probably a bit more neutral, but both are correct and natural.