Learn Hardcore Portuguese (Portugal): Quando defines uma prioridade por dia, a tua rotina torna‑se mais disciplinada. - When you set one priority per day, your routine becomes more disciplined.

You could say Quando tu defines uma prioridade…, but in neutral written Portuguese that tu is usually omitted unless you want to emphasize it (for example, to contrast tu with someone else).

Why is the verb defines and not define or definem?

The verb is definir (to define). In the present tense:

eu defino – I define
tu defines – you define (singular, informal)
ele / ela / você define – he / she / you (formal) define
nós definimos – we define
eles / elas / vocês definem – they / you (plural) define

Because the implied subject is tu (informal "you"), the correct form is defines.

If the sentence were using você instead of tu, it would be:

Quando você define uma prioridade por dia…

Could I say Quando você define instead of Quando defines in Portugal?

Grammatically, yes, but the feel is different:

In Portugal, tu is the normal informal you, and você often sounds distant, formal, or even a bit cold/critical depending on tone and context.
In Brazil, você is usually the default informal you, and tu is regional/colloquial.

So in Portugal, for a friendly, informal tone, Quando defines… (with implied tu) sounds much more natural than Quando você define….

Why is it uma prioridade por dia and not something like uma prioridade cada dia?

Por dia is the most natural way to say per day in Portuguese:

uma prioridade por dia = one priority per day

Other options:

uma prioridade todos os dias – one priority every day (very common, slightly more "everyday speech")
uma prioridade diária – a daily priority (more formal/compact)

Cada dia is not wrong grammatically, but uma prioridade cada dia is unusual. You would more naturally say:

em cada dia tens uma prioridade
todos os dias tens uma prioridade

For this sentence, uma prioridade por dia is the most idiomatic.

Why is it a tua rotina and not just tua rotina?

In European Portuguese, possessives (meu, teu, seu, nosso, etc.) normally come with a definite article:

a tua rotina – your routine
o meu carro – my car
os nossos amigos – our friends

Leaving out the article (tua rotina) is possible but:

It sounds more literary, poetic, or emphatic, not neutral everyday Portuguese.
In normal speech/writing, a tua rotina is the standard form.

So the sentence uses the usual everyday pattern: a + possessive + noun.

What is the difference between tua rotina and sua rotina in Portugal?

In Portugal:

a tua rotina – your routine, addressing someone as tu (informal, close)
a sua rotina – your routine, addressing someone as o senhor / a senhora / você (formal or distant)

So:

Quando defines…, a tua rotina… – informal, friendly, "you" = tu
Quando o senhor define…, a sua rotina… – formal, polite "you"

Using a sua rotina while also using tu would sound inconsistent in European Portuguese.

What does torna‑se mean exactly, and why is it reflexive?

Torna‑se comes from tornar‑se, which means to become in this reflexive form:

tornar = to turn, to make something become something else
tornar‑se = to become (the subject changes state)

So:

a tua rotina torna‑se mais disciplinada
= your routine becomes more disciplined

The reflexive pronoun se shows that the subject (a tua rotina) is undergoing the change itself, not changing something else.

Compare:

O chefe torna o trabalho mais difícil. – The boss makes the work harder.
O trabalho torna‑se mais difícil. – The work becomes harder.

Why is it torna‑se and not se torna?

This is a key difference between European and Brazilian Portuguese:

European Portuguese (EP): In main clauses with no special trigger (like a negative word), the clitic pronoun usually goes after the verb (enclisis):
- A tua rotina torna‑se mais disciplinada.
Brazilian Portuguese (BP): In practice, the pronoun almost always goes before the verb:
- A sua rotina se torna mais disciplinada.

Because the sentence is in European Portuguese, torna‑se is the standard order. Writing se torna would sound Brazilian.

Why is disciplinada feminine here?

Adjectives in Portuguese must agree in gender and number with the noun they describe.

rotina is a feminine singular noun (it ends in ‑a, and it is listed as a rotina in the dictionary).
Therefore, its adjective must also be feminine singular: disciplinada.

Other examples:

uma rotina longa – a long routine
rotinas longas – long routines
um dia disciplinado – a disciplined day (masculine)

So a tua rotina torna‑se mais disciplinada is matching feminine singular (rotina / disciplinada).

Could I change the word order to A tua rotina torna‑se mais disciplinada quando defines uma prioridade por dia?

Yes, that is perfectly correct and natural:

Quando defines uma prioridade por dia, a tua rotina torna‑se mais disciplinada.
A tua rotina torna‑se mais disciplinada quando defines uma prioridade por dia.

Both are fine. The meaning is the same; the difference is only which part you want to present first. The comma is used because quando defines uma prioridade por dia is a subordinate clause placed before the main clause.

Why is the present tense used: Quando defines… a tua rotina torna‑se… and not a future tense?

In Portuguese, the present tense is often used for general truths, habits, and cause–effect relationships, where English might also use the present:

Quando fazes exercício, sentes‑te melhor.
When you exercise, you feel better.

Your sentence describes a general rule/habit, so present–present is natural:

Quando defines… a tua rotina torna‑se…

You could use a future tense in more formal or written language:

Quando definires uma prioridade por dia, a tua rotina tornar‑se‑á mais disciplinada.

This version sounds more formal and more explicitly refers to future situations, but the original present tense is the most common in everyday usage.