Expresó una opinión contraria.

Expresó is the third-person singular preterite form of expresar.

• expresar = to express
• expresó = he/she expressed or you (formal) expressed

The preterite is used for a completed action in the past. So this sentence presents the expressing as a finished event.

Spanish often omits subject pronouns when they are not needed.

In expresó, the verb ending already tells you it is third-person singular, so the subject can be understood from context:

• él expresó = he expressed
• ella expresó = she expressed
• usted expresó = you (formal) expressed

Because of that, Expresó una opinión contraria is a normal complete sentence in Spanish.

The accent mark shows the stress and also helps distinguish it from other forms.

• expreso = I express / express (adjective in some contexts)
• expresó = he/she/you formal expressed

In expresó, the stress falls on the last syllable: ex-pre-SÓ.

Without the accent, it would be read differently.

Una is the feminine singular indefinite article, meaning a or an.

• una opinión = an opinion

Spanish usually requires an article where English sometimes does too, and often where English might leave it out in other contexts. Here, una opinión is the normal way to say an opinion.

Also, opinión is a feminine noun, so it takes una, not un.

Because contraria is an adjective describing opinión, and Spanish adjectives often agree with the noun in gender and number.

• opinión is feminine singular
• so the adjective must also be feminine singular: contraria

Compare:

• una opinión contraria
• un argumento contrario
• unas opiniones contrarias

Here contraria means something like:

• contrary
• opposing
• opposite
• against what was said or expected

So una opinión contraria is an opinion that goes against another opinion, position, or general view.

Depending on context, English might translate it as:

• a contrary opinion
• an opposing opinion
• a different/opposite view

In Spanish, adjectives often come after the noun, especially when they are describing or classifying it in a straightforward way.

So:

• una opinión contraria = a contrary/opposing opinion

Putting the adjective before the noun is sometimes possible in Spanish, but it often sounds more literary, emphatic, or less natural here. For this sentence, opinión contraria is the standard order.

Yes, but it means something slightly different.

• Expresó una opinión contraria = He/She expressed a contrary opinion
• Expresó su opinión contraria = He/She expressed his/her contrary opinion

With su, you are making the opinion more definite: it is his/her/your formal opinion. Without su, the sentence simply says that a contrary opinion was expressed.

Yes. It can mean you expressed if you is formal singular (usted).

So expresó can mean:

• he expressed
• she expressed
• you expressed (formal)

Only context tells you which one is intended.

Opinión is pronounced roughly o-pi-NYON.

The accent mark shows that the stress falls on the last syllable:

• o-pi-NIÓN

It also helps signal the correct pronunciation pattern. This is very common in Spanish words ending in -ión, such as:

• nación
• situación
• expresión

Yes, usually a little.

• decir = to say
• expresar = to express

Expresar often sounds a bit more formal, precise, or neutral, especially in writing. It focuses on the act of putting an idea into words.

So:

• Expresó una opinión contraria sounds natural and slightly formal
• Dijo una opinión contraria is less natural in Spanish

A more natural alternative with decir would usually be something like:

• Dijo algo en contra
• Dijo que no estaba de acuerdo

Not by itself in this sentence.

Here, contraria describes the opinion, not the person. It just means the opinion was in opposition to another view.

If you wanted to describe a person as difficult or contrary, Spanish would usually make that clearer in a different structure. In this sentence, the focus is simply on the content of the opinion.