Dejo el reloj dentro del bolso cuando voy al gimnasio.

Because the implied subject is yo (I).

• Dejar (to leave / to put / to let) in the present tense:
  
  • yo dejo – I leave / I put
  • tú dejas – you leave
  • él/ella deja – he/she leaves
• dejo matches yo, so the sentence means: (Yo) dejo el reloj…

Spanish usually drops subject pronouns because the verb ending already shows the subject.

• Dejo can only be yo in this context, so yo is optional.
• You would add yo for emphasis or contrast:
  
  • Yo dejo el reloj dentro del bolso, pero ella no.
    (I leave the watch in the bag, but she doesn’t.)

Here dejar means to leave (something somewhere) or to put and leave it there.

• Dejo el reloj dentro del bolso = I leave / keep my watch inside the bag.

Other common meanings of dejar (not used here):

• Dejar de fumar – to stop smoking
• Dejar pasar – to let (someone) pass
• Dejarlo en casa – to leave it at home

Big difference:

• Dejo el reloj = I intentionally leave the watch somewhere (on purpose).
• Me dejo el reloj (very common in Spain) = I forget the watch (I accidentally leave it behind).

Example:

• Siempre me dejo el reloj en casa.
  I always forget my watch at home.

Spanish uses definite articles more than English.

• El reloj = the watch (a specific watch, usually my watch).
  In this context, it’s understood as my watch, so Spanish doesn’t need mi.

Dejo el reloj… is more natural than Dejo reloj…, which sounds incomplete or incorrect.

You can say dejo mi reloj; it’s correct.

• Dejo el reloj… and Dejo mi reloj… are both fine.
• In everyday speech, Spanish often omits mi when context makes ownership obvious (your own watch).

Both are correct here, but there is a nuance:

• dentro del bolso = explicitly inside the bag.
• en el bolso = in the bag, usually also understood as inside, but slightly less specific.

In practice, for a bag, dentro del bolso and en el bolso sound almost the same; dentro de just emphasizes the interior.

In Spanish, de + el contracts to del.

• dentro de + el bolso → dentro del bolso

This contraction is obligatory (you can’t say dentro de el bolso).

In Spain:

• el bolso = handbag, purse (the thing you carry your stuff in).
• la bolsa = bag in general, especially plastic bags, shopping bags, tote bags, etc.

So:

• Dejo el reloj dentro del bolso = I leave my watch inside my handbag.
• Dejo el reloj dentro de la bolsa = I leave my watch inside the (shopping/plastic) bag.

Grammatical gender in Spanish is mostly arbitrary. It doesn’t have to match the real‑world gender of who uses the object.

• el bolso (masculine) – handbag
• la mano (feminine) – hand
• la nariz (feminine) – nose

You just have to memorize each noun’s gender.

Again, a + el contracts to al in Spanish:

• cuando voy a + el gimnasio → cuando voy al gimnasio

The contraction is required in normal cases.

Because the sentence is about what you do when you go to the gym, not what you do while you are in the gym.

• cuando voy al gimnasio = when I go to the gym (on my way there / as part of that routine).
• cuando estoy en el gimnasio = when I am at the gym (once I’m there).

Both are correct, but they describe different moments.

Spanish uses the present indicative for habitual actions and for many future-related clauses.

Here it describes a habit:

• Dejo el reloj… cuando voy al gimnasio.
  I (always/usually) leave my watch when I go to the gym.

This is the normal way to express routines in Spanish. You don’t need a future tense here.

Yes, completely correct and very natural:

• Dejo el reloj dentro del bolso cuando voy al gimnasio.
• Cuando voy al gimnasio, dejo el reloj dentro del bolso.

Same meaning. The change is only in emphasis/rhythm, not in grammar.

No accent is needed because it follows normal stress rules.

• Words ending in a vowel, n, or s are stressed on the second‑to‑last syllable by default.
• gim-na-sio ends in a vowel (o), so the stress naturally falls on na: gim-NÁ-sio.

Since the stress is regular, there is no written accent.