Uso el cronómetro del celular para estudiar veinticinco minutos y luego descanso cinco.

In Spanish, subject pronouns (yo, tú, él, etc.) are often left out because the verb ending already shows who the subject is.

• Uso = I use
• Descanso = I rest / I take a break

Adding yo is only needed for emphasis or contrast, for example:

• Yo uso el cronómetro, pero él no.
  I use the stopwatch, but he doesn’t.

So the sentence “Uso el cronómetro del celular…” is perfectly natural and normal.

Spanish often uses the simple present to talk about:

• habits or routines
• things you do regularly

English tends to say “I use … to study 25 minutes and then I rest 5” or “I’m using …”, but in Spanish you normally say:

• Uso el cronómetro del celular…
  I use the stopwatch on my phone…

You’d use estoy usando when you want to emphasize that the action is happening right now, at this exact moment:

• Ahora mismo estoy usando el cronómetro del celular.
  Right now I am using the stopwatch on my phone.

In Spanish, de + el contracts to del:

• de el celular → del celular

So:

• el cronómetro del celular
  literally: the stopwatch of the cell phone
  meaning: the phone’s stopwatch / the stopwatch on my phone

This contraction happens every time you have de + el (singular masculine):

• la pantalla del celular (de + el)
• la cámara del teléfono (de + el)

Both are possible, but there’s a nuance:

• del celular (of the phone)
  Emphasizes that the stopwatch belongs to or is part of the phone.
  → el cronómetro del celular (the phone’s stopwatch)

• en el celular (on the phone)
  Emphasizes the location (the stopwatch is on the phone).
  → el cronómetro en el celular (the stopwatch on the phone)

In everyday speech, “del celular” is very natural for built‑in functions or apps.

This is mainly a regional difference:

• In Latin America, people usually say “celular” or “teléfono celular”.
• In Spain, people usually say “móvil” or “teléfono móvil”.

All of these mean cell phone / mobile phone.

The sentence is using the Latin American word: celular.

Para usually expresses purpose or goal:

• Uso el cronómetro del celular para estudiar…
  I use the stopwatch on my phone *in order to study…*

Por would not be correct here, because you’re not saying “because of studying” or “in exchange for studying”; you’re saying you use it with the purpose of studying.

So:

• para + infinitive = in order to do something
  → para estudiar = in order to study

Numbers from 16 to 29 in Spanish are usually written as single words:

• 16 → dieciséis
• 17 → diecisiete
• 18 → dieciocho
• 19 → diecinueve
• 20 → veinte
• 21 → veintiuno
• 22 → veintidós
• 25 → veinticinco
• 29 → veintinueve

So veinticinco minutos = 25 minutes.

(From 31 upward, they’re normally written as separate words: treinta y cinco, cuarenta y dos, etc.)

In Spanish, when you talk about how long you do something, you often just put the time expression directly after the verb:

• Estudio veinticinco minutos.
  I study for 25 minutes.

You can say:

• Estudio por veinticinco minutos.
• Estudio durante veinticinco minutos.

These are grammatically correct, but in everyday spoken Spanish they often sound less natural than just:

• Estudio veinticinco minutos.

Spanish often omits repeated words when they’re clear from context.
The listener already knows you’re talking about minutes, so you can leave it out:

• …estudiar veinticinco minutos y luego descanso cinco (minutos).

Both forms are correct:

• luego descanso cinco (more concise, very natural)
• luego descanso cinco minutos (explicit, also correct)

It’s similar to English:

• I study 25 minutes and then I rest 5.
  (You don’t need to repeat minutes if it’s clear.)

Minuto is a masculine noun, so:

• el minuto, los minutos
• veinticinco minutos
• descanso cinco minutos

Many nouns ending in -o are masculine (not all, but many), so minuto follows that common pattern.

Yes, you can say:

• …y luego descanso por cinco minutos.
• …y luego descanso durante cinco minutos.

Both are correct and understood.

However, in natural, everyday speech, people will very often just say:

• descanso cinco minutos
  or (when the unit is obvious)
• descanso cinco

So the original “luego descanso cinco” is very idiomatic and natural.

In this context both can work, but there’s a slight nuance:

• luego = then / afterwards (often used in sequences of actions)
• después = afterwards / after that

You could say:

• …y luego descanso cinco.
• …y después descanso cinco.

Both are correct. Luego often feels a bit more like “then” in a sequence:

• Primero estudio, luego descanso.
  First I study, then I rest.

That word order is grammatically possible, but it sounds awkward and unnatural.

The normal order is:

• Uso el cronómetro del celular para estudiar…
  (Subject) + (verb) + (object) + (purpose)

Spanish tends to follow SVO (Subject–Verb–Object) order, and para + infinitive normally comes after the object when expressing purpose.

You can add solo or solamente:

• Uso el cronómetro del celular para estudiar solo veinticinco minutos y luego descanso cinco.
• Uso el cronómetro del celular para estudiar solamente veinticinco minutos y luego descanso cinco.

Both solo and solamente mean “only / just” here.
In speech, solo is more common because it’s shorter.

Both are okay, but slightly different in focus:

• Uso el cronómetro del celular…
  Focus: the stopwatch is a function of the phone (the phone’s stopwatch).

• Uso el cronómetro en mi celular…
  Focus: the stopwatch is located on your phone (on my phone).

In normal conversation, both are widely understood and sound fine. For a Latin American context, “del celular” is very natural in this sentence.