Cuando era adolescente, me costaba seguir las reglas de la casa.

to be

Breakdown of Cuando era adolescente, me costaba seguir las reglas de la casa.

• yo: I
• de: of
• me: me

Questions & Answers about Cuando era adolescente, me costaba seguir las reglas de la casa.

Why is it era instead of fui or estaba?

Era is the imperfect form of ser. It’s used for:

• Ongoing states in the past
• Background information
• Descriptions of what someone “was like” during a period of time

Cuando era adolescente = When I was a teenager (describing a stage/period of life).

Comparisons:

• Cuando fui adolescente – grammatically possible but sounds odd; fui (preterite) suggests a finished, punctual event, and “being a teenager” is not a single event.
• Cuando estaba adolescente – not idiomatic in Spanish. You normally use ser (era) for age/life stages, not estar.

So cuando era adolescente is the natural way to say “when I was a teenager” as a general life phase.

Why is the subject yo not written? How do we know it means “I”?

Spanish commonly omits subject pronouns when the verb form already shows who the subject is.

• era = I was / he was / she was / it was
• me costaba = it was hard for me

In this sentence, context makes it clear:

• The reflexive pronoun me (“to me”) points to the speaker.
• The phrase cuando era adolescente is presented from the speaker’s perspective.

If you really want to emphasize I, you can say:

• Cuando yo era adolescente, me costaba seguir las reglas de la casa.

But in neutral Spanish, yo is usually dropped unless needed for contrast or emphasis.