Learn Hardcore Spanish (Latin America): Esta actividad es muy interesante para mi familia. - This activity is very interesting for my family.

es muy interesante (with ser) describes a general, more permanent characteristic: This activity is (by nature) very interesting.
está muy interesante (with estar) usually suggests a temporary state or how it seems at the moment: This activity is (right now / at this point) very interesting.

In your sentence, we are talking about the general quality of the activity for your family, so es muy interesante is the most natural choice.

Why do we use muy and not mucho before interesante?

Muy is used before adjectives and adverbs:

muy interesante – very interesting
muy rápido – very fast

Mucho is usually used with nouns or verbs:

mucho trabajo – a lot of work
Trabajo mucho. – I work a lot.

Since interesante is an adjective, the correct form is muy interesante, not mucho interesante.

Why is it para mi familia and not a mi familia?

Para and a both translate to for or to in some contexts, but they’re used differently:

para often shows purpose, benefit, or who something is intended for:
- Es interesante para mi familia. – It is interesting for my family (they benefit / are the ones for whom it is interesting).
a is more about direction, movement, or the indirect object of a verb:
- Voy a mi casa. – I’m going to my house.
- Les explico la actividad a mis alumnos. – I explain the activity to my students.

In your sentence, you are saying the activity is interesting for your family, so para mi familia is the natural choice.

Why doesn’t mi in mi familia have an accent, while I sometimes see mí with an accent?

There are two different words:

mi (no accent) = my → a possessive adjective
- mi familia, mi casa, mi libro
mí (with accent) = me → a stressed prepositional pronoun
- para mí, sin mí, de mí

In para mi familia, mi is showing possession (my family), so it never takes an accent. If you said para mí, you’d be saying for me, not for my family.

Why doesn’t interesante change for feminine? Shouldn’t it be something like interesanta?

Adjectives in Spanish follow different patterns:

Many adjectives ending in -o change for gender:
- interesante is not in this group.
- Example: alto / alta (tall)
Adjectives ending in -e usually do not change for gender; they only change for number:
- interesante (singular, masculine or feminine)
- interesantes (plural, masculine or feminine)

So both actividad interesante and libro interesante are correct; the adjective stays interesante for both masculine and feminine singular nouns.

Where is the stress in actividad, and how is it pronounced?

Actividad is pronounced approximately as: [ak-ti-bi-ˈðað] (Latin American pronunciation varies a bit).

Syllables: ac-ti-vi-dad
Stress: on the last syllable (-dad)
The letter v is pronounced like a soft b in Spanish.
The final d in -dad is often softened a lot in many Latin American accents, sometimes sounding almost like -da.

So it will sound something like ak-ti-bi-DAD, with a soft final d.

Can I drop esta and just say La actividad es muy interesante para mi familia?

You can, but it changes the meaning slightly:

Esta actividad = this activity (a specific one, close in context or time)
La actividad = the activity (more generic or already known from context)

If you’re talking about a particular activity you just mentioned or are about to do, esta actividad is more precise. If your listener already knows which activity you mean and it’s the only one in question, la actividad can also work.

Is the word order fixed? Can I say Esta actividad para mi familia es muy interesante?

The basic, most natural order is the original:

Esta actividad es muy interesante para mi familia.

You can move para mi familia toward the middle:

Esta actividad es, para mi familia, muy interesante. (more emphatic / written style)
Para mi familia, esta actividad es muy interesante. (emphasizes for my family)

However, Esta actividad para mi familia es muy interesante sounds awkward and unnatural; Spanish usually keeps the subject – verb – complements order fairly clear in simple sentences like this.

Why is familia treated as singular when it refers to many people?

In Spanish, familia is grammatically singular, even though it refers to a group:

Mi familia es muy grande. – My family is very big.
Esta actividad es muy interesante para mi familia.

So verbs and adjectives that agree with familia are usually in the singular.
In everyday speech, some people might occasionally use plural agreement (e.g. Mi familia son muy unidos), but the standard and most common form is singular: Mi familia es…

Can I leave out muy and just say Esta actividad es interesante para mi familia?

Yes, that is perfectly correct:

Es interesante = It is interesting.
Es muy interesante = It is very interesting.

Adding muy simply intensifies the adjective. Without muy, you are making a more neutral, less emphatic statement.

How would I say “This activity is very interesting for my family and me” in Spanish, and what happens with mi / mí there?

A natural translation would be:

Esta actividad es muy interesante para mi familia y para mí.

Here you use both forms:

mi familia → mi = my (no accent, possessive)
para mí → mí = me (with accent, stressed pronoun after a preposition)

You repeat para before mí to keep the sentence clear and natural in Spanish.