Pensé en ello todo el día.

Pensé is the preterite, which presents the action as a completed whole: I thought about it all day as a finished event.

A learner often compares it with pensaba, the imperfect. The difference is subtle but important:

• Pensé en ello todo el día = I spent the whole day thinking about it, seen as a completed block of time.
• Pensaba en ello = I was thinking about it / I used to think about it, with more emphasis on the ongoing or habitual nature.

Because todo el día gives a clear, completed time frame, pensé sounds very natural here.

The accent mark shows both pronunciation and meaning.

• pensé = I thought (first person singular, preterite)
• pense = not the standard spelling for that form

The accent tells you the stress falls on the last syllable: pen-SÉ.

It also helps distinguish this form from other present subjunctive forms such as piense, which look and function differently.

In Spanish, when pensar means to think about something, it normally uses en:

• pensar en algo = to think about something
• pensar en alguien = to think about someone

So:

• Pensé en ello = I thought about it

Without en, pensar usually means something more like to think / to believe / to have an opinion:

• Pienso que sí = I think so / I think yes

So the preposition is not optional here; it is part of the pattern pensar en.

Ello is a neuter pronoun. It refers to an idea, situation, fact, or something previously mentioned, rather than to a specific masculine noun.

Examples of what ello might refer to:

• a problem
• what someone said
• an entire situation
• an abstract idea

So en ello means about it, where it refers to something neutral or abstract.

This is different from:

• él = he
• ella = she
• ello = it, in a neutral/abstract sense

Not especially. It is correct, but it often sounds formal, written, or slightly literary in everyday conversation.

In spoken Spanish in Spain, people very often say:

• Pensé en eso todo el día

That is usually more natural in casual speech.

So:

• Pensé en ello todo el día = correct, but a bit formal
• Pensé en eso todo el día = very common in conversation

Yes, absolutely. In fact, in everyday speech, eso is often the more natural choice.

A useful rough distinction is:

• ello = more formal, abstract, written style
• eso = very common spoken that / it

So both can work, but the tone changes a little:

• Pensé en ello todo el día = more neutral-formal or literary
• Pensé en eso todo el día = more conversational

Spanish usually omits subject pronouns when the verb form already makes the subject clear.

Here, pensé already tells you the subject is yo:

• pensé = I thought

So Yo pensé en ello todo el día is possible, but normally unnecessary unless you want emphasis, contrast, or clarification.

For example:

• Yo pensé en ello todo el día, pero tú no.
  = I thought about it all day, but you didn’t.

Todo el día is the standard way to say all day.

This structure is very common in Spanish:

• todo el día = all day
• toda la noche = all night
• toda la semana = all week

Using por here would usually sound unnatural for this meaning.

Also note the agreement:

• día is masculine singular
• so we use todo el día, not toda el día

Yes. Spanish word order is more flexible than English word order.

You can say:

• Pensé en ello todo el día
• Todo el día pensé en ello

Both are correct. The difference is mainly focus:

• Pensé en ello todo el día = neutral order
• Todo el día pensé en ello = puts more emphasis on all day

In normal speech, both are possible.

Yes. That is also correct, and it sounds very natural.

• Pensé en ello todo el día = I thought about it all day
• Estuve pensando en ello todo el día = I was thinking about it all day

The second version emphasizes the ongoing process a bit more. It can sound more vivid or more explicitly continuous.

So both work, but:

• pensé = simple, compact, completed event
• estuve pensando = stresses duration or continuous mental activity more strongly

The accent in día shows that the i and a are pronounced in two syllables:

• dí-a

Without the accent, Spanish spelling rules would suggest a different pronunciation pattern.

So:

• día = day, pronounced roughly DEE-ah

This is a very common type of accent mark in Spanish, where it helps show that two vowels are not merged into one syllable.