Learn Hardcore Portuguese (Portugal): Levo sempre a trela e a ração quando saio com a gata. - I always take the leash and the pet food when I go out with the cat.

Speaking at home about going out: you’ll say Levo a trela… (I take it from home to outside).
Speaking later at the park: you’d say Trouxe a trela? (Did I bring it here?). You can add emphasis with levo comigo (I take … with me), but it’s optional here.

Why can the subject eu be omitted?

Portuguese is a “null-subject” language. The verb ending in levo (-o) already tells you the subject is first person singular, so eu is usually omitted unless you want emphasis or contrast, e.g., Eu levo, o João não.

What tense/aspect is levo, and why not estou a levar or costumo levar?

Levo is present indicative and, with sempre, it expresses a habitual action (what you always do). Alternatives:

Costumo levar = I usually take (not as strong as always).
Estou sempre a levar can work, but it often suggests repeated/annoying frequency; the plain present with sempre is the neutral, idiomatic choice for habits.

Can sempre go before or after the verb?

Yes:

Most common: Levo sempre a trela e a ração…
Also fine (slight emphasis on the constancy): Sempre levo a trela e a ração… Avoid pushing sempre to the very end of the clause here; Levo sempre… is the cleanest.

Why the definite article a before trela, ração, and gata? Could it be uma or no article?

a trela: trela is a countable singular noun; you normally need a determiner. A trela implies the specific leash you use. Uma trela would mean “a leash (any leash).”
a ração: ração can be treated as a mass noun. Levo ração = I take some cat food (non‑specific amount). Levo a ração = I take the food we have in mind (e.g., the bag/portion you always bring).
a gata: refers to a specific female cat already known in context. If it’s your pet and you want to say so, see the note on possessives below.

Are these nouns feminine? How can I tell?

Yes: a trela, a ração, a gata are feminine.

-a endings are often feminine: trela, gata.
Nouns ending in -ção are typically feminine: ração (from Latin -tionem).
For animals, masculine/feminine are lexical: o gato (male), a gata (female).

Why gata and not gato? And how do I say “my cat”?

a gata explicitly says the cat is female. Use o gato for a male cat.
To say “my cat” in European Portuguese, use the possessive with the article: a minha gata / o meu gato. In EP, the article is normally kept with possessives (so a minha gata, not just minha gata in neutral statements).

Is com a gata correct? Should it be à gata?

Correct is com a gata. There’s no contraction with com. The form à is the contraction of the preposition a (to) + a (the), unrelated to com.

Examples: com a gata, com o gato; but à rua = to the street.

Why quando saio (present) and not quando sair (future subjunctive)?

For a general/habitual statement (especially with sempre), use the present: Quando saio com a gata, levo…
For a specific future occasion, use the future subjunctive: Quando sair com a gata, levo… (When I go out [on that future occasion], I’ll take…).

Could I use ir instead of sair here?

You can, but there’s a nuance:

sair emphasizes leaving your current place: Quando saio com a gata… is very natural.
ir focuses on going somewhere and usually wants a destination: Quando vou com a gata ao parque… is fine. By itself, Quando vou com a gata… is less idiomatic unless a destination is understood.

What’s the difference between trela, coleira, and arnês/peitoral?

trela: leash/lead (the strap you hold).
coleira: collar (around the neck).
arnês / peitoral: harness (around chest/shoulders). With cats, people often attach the trela to an arnês rather than a coleira.

What exactly does ração refer to? What if it’s wet food or treats?

ração normally means dry pet food/kibble. For other types:

Wet food: comida húmida, patê, lata (tinned food).
Treats: petiscos, snacks, biscoitos (for pets). Examples: Levo ração, Levo a ração da gata, Levo umas latas.

How do you pronounce the sentence in European Portuguese?

Approximate IPA: [ˈlɛvu ˈsẽpɾɨ ɐ ˈtɾɛlɐ i ɐ ʁɐˈsɐ̃w̃ ˈkwɐ̃du ˈsaju kõ ɐ ˈɡatɐ] Notes:

levo [ˈlɛvu], sempre [ˈsẽpɾɨ], trela [ˈtɾɛlɐ], ração [ʁɐˈsɐ̃w̃] (initial r is guttural; final -ão is nasal), quando [ˈkwɐ̃du], saio [ˈsaju], gata [ˈɡatɐ].
The conjunction e is pronounced [i].
com a is often heard as [kõ ɐ] (nasal vowel on com).

If I replace a trela e a ração with a pronoun, what happens?

Both are feminine singular nouns coordinated, so use the feminine plural object pronoun as:

Neutral affirmative main clause (EP enclisis): Levo-as sempre.
With negation (proclisis): Não as levo.
With the original order: Quando saio com a gata, levo-as sempre.

Can the quando-clause go first?

Yes: Quando saio com a gata, levo sempre a trela e a ração. When the quando-clause comes first, add a comma before the main clause.