Yo debería haber guardado el martillo antes, porque ahora no sé dónde está.

No. In Spanish, the subject pronoun is often omitted because the verb already shows who is doing the action.

So these both work:

• Yo debería haber guardado el martillo antes...
• Debería haber guardado el martillo antes...

Including yo adds a bit of emphasis, as if the speaker is reflecting on their own responsibility: I should have put the hammer away.

This is a very common Spanish structure:

• debería = should
• haber guardado = have put away / have stored

Together, debería haber guardado means should have put away.

Grammar-wise:

• debería is the conditional form of deber
• haber guardado is the perfect infinitive: haber
  • past participle

This combination is used to talk about something that would have been the right thing to do in the past, often with regret.

Because debería haber guardado is the most straightforward way to say should have put away.

Compare:

• debería haber guardado = I should have put it away
• debía guardarlo = I was supposed to put it away / I should put it away
• debí haberlo guardado = also I should have put it away, but often sounds a bit stronger or more direct

For many learners, debería haber + participle is the safest pattern for should have done.

Because after debería, Spanish uses the infinitive form, not another conjugated verb.

So:

• debería haber guardado = correct
• debería he guardado = incorrect

Only the first verb is conjugated here:

• debería = conjugated
• haber = infinitive
• guardado = past participle

This is similar to English should have saved/stored.

Because the sentence refers to a completed action in the past.

• guardar = to put away / to store
• guardado = put away / stored

After haber, Spanish uses a past participle:

• haber guardado
• haber comido
• haber salido

So debería haber guardado literally follows the pattern should have + past participle.

Guardar often means to put away, to store, to keep, or to put something back where it belongs.

That makes it a very natural verb for objects like tools:

• guardar el martillo
• guardar las llaves
• guardar la ropa

It suggests not just moving the hammer, but putting it away properly.

Because Spanish usually uses an article with a specific countable noun.

Here, el martillo means the hammer, referring to a particular hammer the speaker has in mind.

English sometimes omits articles in places where Spanish does not, but here Spanish normally needs el.

Because antes is being used on its own as an adverb, meaning earlier or before.

• antes = before / earlier

You use antes de when something comes after it:

• antes de salir = before leaving
• antes de la cena = before dinner

In this sentence, antes stands alone, so de is not needed.

Because porque is the conjunction meaning because.

• porque = because
• por qué = why

So:

• No fui porque estaba cansado. = I didn’t go because I was tired.
• ¿Por qué no fuiste? = Why didn’t you go?

In your sentence, the speaker is giving a reason, so porque is correct.

Because it appears in an indirect question.

• No sé dónde está. = I don’t know where it is.

Even though this is not a direct question with question marks, it still contains the idea of a question: where is it?

Compare:

• ¿Dónde está? = direct question
• No sé dónde está. = indirect question

But without the accent, donde is usually a relative word:

• El lugar donde está = the place where it is

Because Spanish uses estar for location.

• El martillo está en la caja. = The hammer is in the box.
• No sé dónde está. = I don’t know where it is.

Even if the object is always kept somewhere, Spanish still normally uses estar for physical location.

Ser would not sound natural here.

Because Spanish keeps the normal indirect-question structure after verbs like saber.

• ¿Dónde está? = Where is it?
• No sé dónde está. = I don’t know where it is.

The subject is not stated because Spanish often leaves it out when it is understood. If you wanted, you could make it more explicit:

• No sé dónde está el martillo.

But in your sentence, el martillo has already been mentioned, so leaving it out is more natural.

They help with pronunciation and, in some cases, distinguish one word from another.

• debería: the accent shows the stress falls on -rí-
• sé: distinguishes I know from se
• dónde: marks it as an interrogative/indirect interrogative word
• está: shows the stress and distinguishes it from esta

So the accents are not optional here; they are part of the correct spelling.

Yes. Ahora adds the idea of now, making the contrast clearer:

• ...porque ahora no sé dónde está. = ...because now I don’t know where it is.

Without ahora, the sentence still works:

• ...porque no sé dónde está.

Adding ahora makes the consequence feel more immediate.