Encontré el diario ayer.

Encontré is the 1st person singular preterite form of encontrar.

• encontré = I found
• It is used for a completed action in the past

That fits this sentence because the finding happened at a specific past time: ayer.

Compare:

• encuentro = I find / I am finding
• encontraba = I was finding / I used to find
• he encontrado = I have found

With ayer, the preterite is the normal choice.

Spanish often omits subject pronouns when the verb ending already makes the subject clear.

In encontré, the ending -é tells you the subject is I. So yo is not necessary.

You could say Yo encontré el diario ayer, but that usually adds:

• emphasis
• contrast
• or extra clarity

For example:

• Yo encontré el diario, no Ana. = I found the newspaper/diary, not Ana.

So the version without yo is completely normal and natural.

The accent mark shows that the stress falls on the last syllable: en-con-tré.

It is also the normal spelling pattern for the yo preterite of regular -ar verbs:

• hablé
• compré
• encontré

So the accent is not optional. It is part of the correct written form.

El is the definite article, so it means the speaker is referring to a specific diary/newspaper.

• el diario = the diary/newspaper
• un diario = a diary/newspaper

So Encontré el diario ayer suggests that the listener already knows which one is meant, or that it is identifiable from context.

Because diario is a masculine singular noun, so it takes el.

• el diario
• plural: los diarios

This is a matter of grammatical gender, not biological gender. Many nouns in Spanish are simply classified as masculine or feminine.

Because the personal a is generally used with people or sometimes pets/personified beings, not ordinary things.

Compare:

• Encontré a María ayer. = I found María yesterday.
• Encontré el diario ayer. = I found the diary/newspaper yesterday.

Since el diario is a thing, no personal a is used.

Yes. Spanish word order is flexible, especially with time expressions.

All of these are possible:

• Encontré el diario ayer.
• Ayer encontré el diario.
• Encontré ayer el diario.

The most neutral options are usually:

• Encontré el diario ayer
• Ayer encontré el diario

Putting ayer first often gives a little more focus to when it happened.

In Spanish from Spain, when you use a finished time expression like ayer, the normal choice is the preterite:

• Encontré el diario ayer.

Using he encontrado with ayer sounds unnatural in standard usage.

In Spain, he encontrado is more natural with time periods that are still connected to the present, such as:

• hoy
• esta semana
• este mes

For example:

• Hoy he encontrado el diario.

So ayer strongly points you toward encontré.

It is pronounced with the stress on the last syllable: en-kon-TRÉ.

A rough guide for the full sentence is: en-kon-TRÉ el dya-RYO a-YER

A few helpful points:

• r in diario is a tapped Spanish r
• ll does not appear here, so nothing tricky there
• ay in ayer sounds like the beginning of English eye

The most important stress point is encontré, not ENcontré.

Not always. Depending on context, diario can mean:

• diary / journal
• newspaper

In Spain, diario is very commonly used for newspaper, and periódico is also very common.

So the exact meaning depends on context. In this sentence, you should use the meaning that was already given to you, but it is useful to know that diario can have more than one meaning.

Yes, it is completely natural.

It is short, clear, and idiomatic:

• Encontré = correct past tense
• el diario = normal noun phrase
• ayer = natural time expression

A native speaker in Spain would understand it immediately. It sounds like a simple everyday statement.