Learn Hardcore Spanish (Latin America): Mi tío camina por la calle cada día.

In Spanish, accent marks serve two main purposes: indicating irregular stress and marking a hiatus (two vowels pronounced in separate syllables). Tío has the accent on the í because:

The vowels i and o form a hiatus rather than a diphthong, so they must be pronounced separately (TÍ-o).
Without the accent, standard stress rules would place stress on the penultimate syllable and might treat io as a diphthong.

What tense is camina, and why is it used for a habitual action?

Camina is the third-person singular of the present indicative from the verb caminar (“to walk”). In Spanish, the simple present tense frequently describes:

Habitual or repeated actions (e.g., “he walks every day”).
General truths.
So camina naturally conveys “he walks” as a regular activity, without needing a progressive form.

Why is there no subject pronoun like él before camina?

Spanish verb endings already identify the subject. The -a ending in camina tells you the subject is él/ella/usted. Adding él (he) would be redundant unless you want to emphasize or clarify who you’re talking about.

Why is the preposition por used in por la calle instead of en?

Por indicates movement through, along, or by a place (“walking along the street”).
En would focus on location rather than the path (“in/on the street”).
Since the sentence describes moving along the street, por is the correct choice.

Why is the definite article la used before calle?

Spanish generally requires an article before a noun in general or habitual statements. Por la calle (“through the street”) is idiomatic. Omitting the article (por calle) sounds unnatural and ungrammatical.

Is cada día the same as todos los días, and can I use them interchangeably?

Yes. Both mean “every day.”

Cada día emphasizes viewing each day individually (“each day”).
Todos los días emphasizes the repetition (“all the days”).
In practice, either form works perfectly for a habitual action.

Why is cada día written as two separate words? Isn’t “cadadía” one word?

In Spanish, cada is an adjective meaning “each,” and día is the noun “day.” Adjectives and the nouns they modify are always separate: cada día, cada semana, etc. There is no word cadadía.

Could I use the present progressive (está caminando) instead of camina here?

You could say Mi tío está caminando por la calle cada día, but that changes the nuance:

Está caminando (present progressive) highlights the action in progress at the moment or during a specific period.
Camina (simple present) focuses on a habitual routine.
For everyday habits, Spanish prefers the simple present.