No era mi intención.

Both are possible, but they do not sound exactly the same.

• Era is the imperfect of ser. It presents your intention as a past state of mind or background idea.
• Fue is the preterite. It sounds more definite and more linked to one completed event.

So No era mi intención often feels a bit softer and more explanatory, especially when you are clarifying or apologising after something happened.

Examples:

• No era mi intención molestarte. = softer, explanatory
• No fue mi intención molestarte. = also correct, a bit more definite

Because mi intención is a noun, and Spanish normally uses ser to identify or define something with a noun.

Compare:

• Era un error.
• Fue una sorpresa.
• No era mi intención.

Using estar here would be incorrect, because you are not talking about location or temporary condition. You are saying what something was or was not in terms of identity or purpose.

Spanish often leaves that kind of subject unstated when it is obvious from context.

English needs a subject, so we say It wasn't my intention or That wasn't my intention. Spanish does not need an equivalent dummy subject here. The listener understands it from the situation.

So No era mi intención is a complete, natural sentence on its own.

In Spanish, basic negation usually goes directly before the conjugated verb.

So:

• Era mi intención. = affirmative
• No era mi intención. = negative

This is the normal word order in simple negative sentences.

Because mi without an accent is the possessive adjective meaning my.

• mi intención = my intention

Mí with an accent is a pronoun used after prepositions:

• para mí = for me
• de mí = of me
• a mí = to me

So in this sentence, only mi is correct.

In Spanish, a possessive adjective usually replaces the article.

So you say:

• mi intención not
• la mi intención

Also, intención is feminine, but mi does not change for gender in the singular.

Compare:

• mi libro = masculine
• mi intención = feminine

If you used the noun without a possessive, then it would be:

• la intención

The accent mark shows where the stress goes: intención.

This matters because Spanish spelling has default stress rules. A word ending in n, s, or a vowel is normally stressed on the second-to-last syllable. Without the written accent, the stress would not fall where it should.

So the accent tells you clearly that the stressed syllable is the last one:

• in-ten-CIÓN

A good approximation for most of Spain is:

noh EH-ra mee in-ten-THYON

IPA: /no ˈe.ɾa mi in.tenˈθjon/

A few useful details:

• era has a single tapped r, not a strong rolled rr
• intención is stressed on the last syllable
• in most of Spain, c before i sounds like th in think, so -ción sounds roughly like -thyon

It is neutral and very common. You can use it in both informal and formal situations.

It works well when:

• apologising
• clarifying what you meant
• softening the impact of something you said or did

For example, it can sound polite and natural in conversation, messages, or even more formal speech.

Yes, very often.

A common pattern is:

• No era mi intención + infinitive

Examples:

• No era mi intención ofenderte.
• No era mi intención hacerte daño.
• No era mi intención interrumpir.

You can also use a clause:

• No era mi intención que te sintieras mal.

A simple way to think about it:

• infinitive when you say the action directly
• que + subjunctive when you introduce a full clause or someone else’s reaction

They are similar, but not identical.

• No era mi intención focuses on your purpose or what you meant.
• No tenía intención de... focuses more on not planning or not deliberately meaning to do something.

Compare:

• No era mi intención ofenderte. = offending you was not what I meant to do
• No tenía intención de ofenderte. = I had no intention of offending you / I was not planning to offend you

Both are common, but No era mi intención often sounds especially natural when you are explaining yourself after an unwanted result.