Anoche me puse a buscar mi saco de dormir y descubrí que estaba todavía mojado.

In this structure, ponerse a + infinitive means “to start / to begin to do something.”

• poner = to put
• ponerse (reflexive) can mean “to get / to become” or “to start doing something”
• me is the reflexive pronoun for yo, so me puse = “I got / I started.”

So me puse a buscar literally is “I put myself to look for” → idiomatically, “I started looking for / I began to look for.”

Yes.

All three are natural:

• me puse a buscar
• empecé a buscar
• comencé a buscar

Differences are small and mostly about style:

• me puse a buscar often sounds a bit more spontaneous or informal, like “I went and started looking for…”
• empecé a buscar and comencé a buscar are a little more neutral/formal: “I began to look for…”

In everyday Latin American Spanish, any of these would be fine in this sentence.

Spanish could grammatically use either, but the nuance changes:

• mi saco de dormir = my sleeping bag (explicit, personal possession)
• el saco de dormir = the sleeping bag (more generic; could be shared or just “the” one already known in context)

In English, you’d normally say “my sleeping bag” here, so mi saco de dormir is the most natural choice.

Saco de dormir literally means “bag for sleeping” and is widely understood as “sleeping bag.”

Regional options:

• saco de dormir – common and widely understood
• bolsa de dormir – also used in several Latin American countries

For learning purposes, saco de dormir is a safe, neutral term.

They describe different types of past events:

• descubrí (preterite) = a completed action at a specific moment: “I discovered / I found out.”
• estaba (imperfect) = a background state that was already in progress: “it was (in the state of being) wet.”

So at one point in time, you discovered a condition that was already true.
That contrast (sudden discovery vs. ongoing state) is why the mix preterite + imperfect is used.

Spanish generally uses:

• estar for temporary states or conditions (wet, tired, dirty, open, closed, etc.)
• ser for more permanent traits or identities (tall, Mexican, intelligent, wooden, etc.)

Being wet is a temporary condition, so:

• estaba mojado = “it was wet (at that time).”

Era mojado would sound incorrect in this context.

Yes, all of these are possible:

• …descubrí que estaba todavía mojado.
• …descubrí que todavía estaba mojado.
• …descubrí que estaba mojado todavía.

They all mean “I discovered that it was still wet.”

Very slight naturalness/preferences:

• todavía estaba mojado is probably the most common-sounding version.
• estaba todavía mojado gives a touch more emphasis to todavía.
• estaba mojado todavía is also fine, often conversational.

For everyday use, any of the first two is ideal.

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

• el saco de dormir → masculine singular
• So the adjective must also be masculine singular: mojado.

Compare:

• el saco está mojado (masc. sing.)
• la camisa está mojada (fem. sing.)
• los sacos están mojados (masc. plural)
• las camisas están mojadas (fem. plural)

Since we’re talking about el saco de dormir, mojado is correct.

After poner(se) a, the next verb must be in the infinitive:

• ponerse a + infinitive → me puse a buscar, me puse a leer, me puse a cocinar

You can’t conjugate that second verb there:

• ❌ me puse a busqué
• ✔️ me puse a buscar

It’s like English: “I started to look for”, not “I started looked for.”

They’re very close in meaning:

• anoche = “last night”
• ayer por la noche = “yesterday evening / yesterday night”

In most everyday contexts, they refer to the same time period.
Anoche is shorter and very common in speech, which is why it appears here.

Me puse a + infinitive is neutral and common in spoken Spanish. It’s not slang.

Alternatives:

• empecé a buscar – very common and neutral
• comencé a buscar – fine, sometimes feels a bit more formal or written

Talking about your day, me puse a buscar or empecé a buscar are both very natural.

Yes, this is also correct:

• Anoche me puse a buscar mi saco de dormir y descubrí que estaba todavía mojado.
• Anoche me puse a buscar mi saco de dormir y descubrí que mi saco de dormir todavía estaba mojado.

The original avoids repeating mi saco de dormir by just using estaba with the subject understood.
The version that repeats the noun is a bit heavier, but perfectly grammatical and clear—especially useful when you’re still getting used to pronouns and implicit subjects.