Learn Hardcore Portuguese (Portugal): Uso uma aplicação para praticar respiração lenta antes de adormecer. - I use an app to practise slow breathing before falling asleep.

uso already tells you it’s I (first person singular).
Eu uso is also correct, but it normally adds some emphasis, like:
- Eu uso uma aplicação… = I (as opposed to other people) use an app…

So both are grammatically correct. The sentence without eu is just more neutral and more typical in everyday Portuguese.

What tense is uso here, and does it mean “I use” or “I am using”?

uso is the present indicative tense.

In Portuguese, the simple present often covers both:

habitual actions:
- Uso uma aplicação para praticar respiração lenta.
  = I use an app (as a routine / habit).
actions happening now (in the right context):
- Agora não posso falar, uso uma aplicação.
  = I’m using an app right now.

In your sentence, the most natural interpretation is habitual: “I (normally) use an app to practise slow breathing before falling asleep.”

Why is it uma aplicação and not something like um aplicativo or just app?

You specified Portuguese from Portugal (European Portuguese), where:

aplicação is the standard word for a software application.
It is feminine (hence uma aplicação), partly because nouns ending in -ção are almost always feminine.

In Brazilian Portuguese, people very often say:

um aplicativo or um app for “an app”.

In European Portuguese you can also hear uma app in informal speech, especially among younger speakers, but uma aplicação is perfectly standard and natural.

Why is it uma aplicação and not a aplicação? What would a aplicação change?

Uso uma aplicação… = I use an app…
This is indefinite: any app that serves that purpose. You’re introducing the idea, not referring to a specific, previously mentioned app.
Uso a aplicação… = I use the app…
This would sound like you and your listener already know which exact app you’re talking about (maybe you mentioned it before, or there is a specific official app).

So uma aplicação is the natural choice if you’re just stating a general habit and the specific app is not important.

Why is it para praticar and not just para praticar a respiração lenta or para praticar a respirar lentamente?

para praticar respiração lenta literally = “in order to practise slow breathing.”

Several structures are possible:

para praticar respiração lenta
- Focuses on the activity as a noun phrase (respiração lenta).
- Sounds a bit like “to practise slow breathing (as a technique).”
para praticar a respiração lenta
- More specific or slightly more concrete, like “to practise the slow breathing (technique).”
- The definite article a makes it sound a bit more like a known method.
para praticar respirar lentamente
- Uses the verb respirar (“to breathe”) instead of the noun respiração.
- This is also possible and maybe a bit more colloquial: “to practise breathing slowly.”

All three are grammatically fine. The original sentence is neutral and acceptable; in more natural speech, many people might prefer something like:

Uso uma aplicação para praticar a respiração lenta.
Uso uma aplicação para praticar respirar lentamente.

Why is respiração lenta and not respiração devagar or respirar lento / respirar lentamente?

There are a few different things here:

respiração lenta
- respiração is a noun: “breathing”.
- lenta is an adjective: “slow”.
- Literally: “slow breathing.”
- This sounds like a technique or exercise, which fits the context very well.
respiração devagar
- devagar is usually an adverb (“slowly”), not an adjective.
- Using it right after a noun (respiração devagar) sounds odd; it’s much more natural with a verb: respirar devagar (“to breathe slowly”).
respirar lento / respirar lentamente
- Here respirar is the verb “to breathe”.
- respirar lentamente / respirar devagar is perfectly good Portuguese for “to breathe slowly.”
- respirar lento is less standard; you’d usually say respirar devagar or respirar lentamente.

So in this sentence, respiração lenta is natural because you are “practising” a type of breathing as an exercise.

Why is lenta placed after respiração? Could it be lenta respiração?

In Portuguese, the normal position of adjectives is after the noun:

respiração lenta = slow breathing
aplicação móvel = mobile app

Adjectives can come before the noun, but that usually adds some nuance (more literary, emotional, or stylistic), and not all combinations sound natural.

lenta respiração is grammatically possible, but it sounds poetic or literary, not like a neutral everyday sentence.

In normal speech, you should say respiração lenta.

Why is it antes de adormecer and not just antes adormecer?

When antes (“before”) is followed by a verb or a noun, European Portuguese normally requires the preposition de:

antes de adormecer = before falling asleep
antes de jantar = before having dinner
antes de a reunião / antes da reunião = before the meeting

So:

antes adormecer is incorrect.
You always need antes de
- infinitive (or noun) in this kind of structure.

What is the difference between adormecer and dormir? Could I say antes de dormir?

Yes, you could say antes de dormir, and it would still be natural.

Difference:

adormecer = “to fall asleep” (the moment of going from awake to asleep, the transition).
- antes de adormecer = before I fall asleep (before I drift off).
dormir = “to sleep” (the state or act of sleeping).
- antes de dormir = before sleeping / before I go to sleep.

In daily conversation, both are very close in meaning here. antes de adormecer slightly emphasizes the moment you are about to fall asleep; antes de dormir is a bit more general.

Why is adormecer in the infinitive? Why not something like antes que eu adormeça?

Portuguese has two common patterns:

antes de
- infinitive
- antes de adormecer
- Literally “before to fall asleep” / “before falling asleep.”
- Very frequent and neutral, especially when subject is clear from context.
antes que
- verb in the subjunctive
- antes que eu adormeça
- Grammatically correct, but in this simple sentence, it sounds heavier or more formal and is less common.

For a straightforward, everyday sentence like yours, antes de adormecer is definitely the more natural choice.

Why is para used here? Could we use por instead of para?

In this sentence, para expresses purpose:

Uso uma aplicação para praticar…
= I use an app in order to practise…

In European Portuguese:

para is the normal preposition for purpose / goal / intention.
por is more for cause, reason, means, duration, movement through:
- Fiz isso por ti. = I did this for you (because of you / on your behalf).
- Andámos por Lisboa. = We walked around Lisbon.

So:

Uso uma aplicação por praticar respiração lenta is incorrect in this sense.
You should use para here, not por.

Is the word order fixed? Could I say Uso uma aplicação antes de adormecer para praticar respiração lenta?

Portuguese word order is relatively flexible, especially with prepositional phrases like para… and antes de….

All of these are possible and grammatical:

Uso uma aplicação para praticar respiração lenta antes de adormecer.
Uso uma aplicação antes de adormecer para praticar respiração lenta.

Differences are mostly about emphasis and flow:

Putting para praticar respiração lenta earlier highlights the purpose of the app first.
Putting antes de adormecer earlier highlights the time when you use it.

The original order is very natural and probably the most neutral.

Is the sentence formal, informal, or neutral? How would this sound in casual conversation in Portugal?

The sentence is neutral: clear, standard Portuguese, suitable in most contexts (talking to friends, a doctor, in a survey, etc.).

In casual spoken European Portuguese, you might also hear slightly different versions like:

Uso uma app para praticar a respiração lenta antes de adormecer.
Costumo usar uma aplicação para praticar a respiração lenta antes de adormecer.
(adds the idea of “I usually / I tend to use…”)

But your sentence as given sounds perfectly natural and correct in Portugal.