Learn Hardcore Spanish (Latin America): Cuando era adolescente, me costaba seguir las reglas de la casa. - When I was a teenager, it was hard for me to follow the house rules.

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.

What does me costaba literally mean, and why is me there?

Literally:

costar = to cost
me costaba ≈ “it was costing me” or “it was difficult for me”

In this sense, costar works like gustar-type verbs:

Me costaba seguir las reglas = “Following the rules was hard for me.”

Structure:

costaba – 3rd person singular (the thing that is hard = “seguir las reglas”).
me – indirect object pronoun (“to me/for me”).

So grammatically, the idea is:
(Seguir las reglas) me costaba.
“(Following the rules) was hard for me.”

Why costaba and not costó or me cuesta?

me costaba – imperfect past: it used to be hard for me, or it was (generally) hard during that period.
me costó – preterite past: it was hard for me (on a specific occasion) or as a completed event.
me cuesta – present: is hard for me (now).

The sentence talks about a repeated or ongoing difficulty in adolescence, not a one-time struggle, so the imperfect (costaba) is correct.

Why not just say era difícil seguir las reglas instead of me costaba seguir las reglas?

You can say:

Cuando era adolescente, era difícil seguir las reglas de la casa.

That’s correct, but there’s a nuance:

era difícil seguir las reglas – more neutral/impersonal: “it was difficult to follow the rules.”
me costaba seguir las reglas – more personal, highlights the speaker’s own struggle: “I had a hard time following the rules.”

Me costaba focuses on how the action affected me, not just on the general difficulty level.

Why is seguir used instead of obedecer or another verb?

Seguir las reglas is a very common collocation in Spanish and matches English “follow the rules.”

seguir las reglas – follow the rules (very common, neutral).
obedecer las reglas / obedecer a mis padres – obey the rules / obey my parents (a bit more formal or explicit about obedience).
cumplir las reglas – comply with the rules, observe the rules.

In everyday speech about family rules, seguir las reglas sounds the most natural and conversational.

Why is it las reglas de la casa and not something like las reglas en la casa?

Both are possible, but they’re not quite the same:

las reglas de la casa – literally “the rules of the house”; this is the standard way to say “house rules” or “the household rules.”
las reglas en la casa – “the rules in the house”; focuses more on location and sounds less idiomatic as a fixed expression.

In most contexts where English says “the house rules” or “the rules at home,” Spanish will say las reglas de la casa.

Why do we need the article la in de la casa?

Spanish usually keeps the definite article in these “of the X” kinds of phrases:

las reglas de la casa – the rules of the house
las normas de la escuela – the rules of the school

Saying las reglas de casa (without la) is possible in some expressions, but:

las reglas de casa sounds more like “rules for at home / home rules (in general).”
las reglas de la casa points to a specific house (the speaker’s home).

Since we’re clearly talking about one particular home, de la casa is the natural choice.

Why is cuando followed by era (indicative) and not the subjunctive?

In time clauses with cuando, the mood depends on meaning:

If it’s about a real, past, completed or habitual situation → indicative.
If it’s about a future/uncertain action from the speaker’s point of view → subjunctive.

Examples:

Cuando era adolescente, me costaba…
Past, real, background → indicative (era).
Cuando sea adolescente, quizás le cueste seguir las reglas.
Future, not yet real → subjunctive (sea).

In your sentence, we’re describing a real past period of life, so indicative (era) is correct.

Could the sentence be Cuando era adolescente, me costó seguir las reglas de la casa? What’s the difference?

Yes, that’s grammatically correct, but the nuance changes:

me costaba (imperfect) – ongoing or habitual difficulty:
- “When I was a teenager, I had a hard time (in general) following the house rules.”
me costó (preterite) – a specific or summarized difficulty:
- “When I was a teenager, it was hard (as a whole / on a particular occasion) to follow the house rules.”

In most contexts talking about teenage years as a phase characterized by that difficulty, me costaba is the more natural choice.

Could we change Cuando era adolescente to something like De adolescente? Does it mean the same?

Yes, you can say:

De adolescente, me costaba seguir las reglas de la casa.

This is very common and means essentially the same as:

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

Nuance:

Cuando era adolescente – slightly more explicit, sounds a bit more like storytelling/background.
De adolescente – more compact and colloquial.

Both are correct in Latin American Spanish.

Is there any difference in meaning if I say A mí me costaba seguir las reglas de la casa?

Me costaba seguir las reglas de la casa. – neutral: “It was hard for me to follow the house rules.”
A mí me costaba seguir las reglas de la casa. – adds emphasis on “me”:

It suggests a contrast:

A mí me costaba seguir las reglas de la casa, pero a mis hermanos no.
“I had trouble following the house rules, but my siblings didn’t.”

So a mí is optional and used mainly when you want to stress I (as opposed to others).