Estaba en la biblioteca y, de pronto, se me olvidó la contraseña.

Estaba is the imperfect, which is often used to give background information or describe an ongoing situation in the past.

So in Estaba en la biblioteca, the speaker is setting the scene: they were in the library when something happened.

If you said Estuve en la biblioteca, it would sound more like I was in the library / I spent time in the library as a completed event, not just background.

A useful contrast is:

• Estaba en la biblioteca cuando... = I was in the library when...
• Estuve en la biblioteca toda la tarde = I was in the library all afternoon

De pronto means suddenly or all of a sudden.

It introduces the unexpected event:

• Estaba en la biblioteca y, de pronto, se me olvidó la contraseña.

Other common Spanish expressions with a similar meaning are:

• de repente
• de golpe (more informal in some contexts)

Both are possible, but they feel different.

Se me olvidó la contraseña is a very common and natural way to say I forgot the password, especially when the idea is that it slipped my mind.

It is less direct and often sounds more like the forgetting was accidental or just something that happened.

By contrast:

• Olvidé la contraseña = I forgot the password

This is grammatically fine, but it sounds a bit more direct and less idiomatic in many everyday situations.

A very literal way to think of se me olvidó la contraseña is:

• the password forgot itself on me
• or better: the password slipped my mind

That literal image helps explain the grammar.

In this structure:

• me means to me
• se is part of the verb construction olvidarse

So se me olvidó la contraseña literally works like:

• the password was forgotten to me
• or more naturally: the password slipped my mind

The important thing for learners is this:

• me shows who is affected
• the thing forgotten, la contraseña, behaves like the subject of the verb

This same pattern appears with other people:

• se te olvidó = you forgot
• se le olvidó = he/she/you forgot
• se nos olvidó = we forgot

Because in se me olvidó la contraseña, the verb agrees with la contraseña, not with me.

The grammatical subject is la contraseña, which is singular, so the verb is singular:

• se me olvidó la contraseña

If the forgotten thing were plural, the verb would also be plural:

• se me olvidaron las llaves = I forgot the keys

This is one of the most important things to notice in this structure: the verb matches the thing forgotten, not the person who forgot it.

Spanish often uses the definite article (el, la, los, las) where English might prefer a possessive like my.

So:

• se me olvidó la contraseña

very naturally means:

• I forgot my password

The context already tells us whose password it is, because of me.

You could say mi contraseña, but it would usually add emphasis or clarity:

• Se me olvidó mi contraseña, no la tuya.

In normal everyday Spanish, la contraseña is the most natural choice here.

Yes. De pronto is quite flexible.

For example:

• Estaba en la biblioteca y, de pronto, se me olvidó la contraseña.
• De pronto, se me olvidó la contraseña.
• Se me olvidó la contraseña de pronto.

The first version is very natural because de pronto interrupts the background description and introduces the sudden event.

The commas show it as a kind of parenthetical expression. In informal writing, people may not always punctuate it carefully, but the meaning stays the same.

Yes. That is also very natural, especially in Spain.

• Se me olvidó la contraseña = preterite; a completed past event
• Se me ha olvidado la contraseña = present perfect; often used in Spain for something recent or still relevant now

Since forgetting a password usually matters in the present, se me ha olvidado la contraseña is extremely common in Spain.

Both are correct. The difference is mainly one of tense choice and regional preference.

No. Both are useful, but they are not always used in exactly the same way.

Use olvidar when you want a straightforward direct verb:

• Olvidé el libro en casa.
• Nunca olvidaré ese día.

Use olvidarse very often for everyday accidental forgetting:

• Se me olvidó el libro.
• Se te ha olvidado llamar.

A good rule for learners is:

• for something slipped my mind, olvidarse is very common
• for a more direct to forget, olvidar also works

A very useful pattern is:

• se + indirect object pronoun + verb + thing forgotten

Examples:

• Se me olvidó la contraseña.
• Se te olvidaron las llaves.
• Se nos olvidó comprar pan.
• Se le ha olvidado tu nombre.

The indirect object pronouns are:

• me = to me
• te = to you
• le = to him/her/you
• nos = to us
• os = to you all
• les = to them/you all

This pattern is worth memorising because it is extremely common in everyday Spanish.