Las matemáticas me cuestan menos cuando estudio con mi amiga.

In everyday Spanish, las matemáticas is the normal way to say maths / mathematics as a school subject or area of study.

Even though English often treats math as singular, Spanish commonly uses the plural form:

• Las matemáticas son difíciles.
• Me gustan las matemáticas.

You can sometimes see la matemática, but that is more formal or abstract, often referring to mathematics as a discipline rather than the school subject in ordinary conversation.

Because the subject is las matemáticas, which is grammatically plural.

The verb costar has to agree with its subject:

• La física me cuesta. → singular
• Las matemáticas me cuestan. → plural

So cuestan matches las matemáticas.

Here, costar means to be difficult for someone, to take effort, or to be hard.

So:

• Las matemáticas me cuestan = Maths are hard for me
• more literally, Maths cost me effort

This is a very common use of costar in Spanish.

Examples:

• Me cuesta levantarme temprano. = It’s hard for me to get up early.
• Le cuesta hablar en público. = It’s hard for him/her to speak in public.

Me means to me / for me.

In this structure, Spanish marks the person affected by the difficulty with an indirect object pronoun:

• me = to me
• te = to you
• le = to him/her/you formal
• nos = to us
• os = to you all
• les = to them/you all formal

So:

• Las matemáticas me cuestan = Maths are difficult for me
• Las matemáticas te cuestan = Maths are difficult for you

Spanish does not usually say this the way English does with I find maths difficult. Instead, it uses a structure closer to Maths are hard for me.

Menos means less or less so.

In this sentence, the comparison is understood from context. It means something like:

• Maths is less difficult for me when I study with my friend
• I struggle less with maths when I study with my friend

Spanish often leaves the comparison implicit when it is obvious enough.

If you wanted to make it explicit, you could say something like:

• Las matemáticas me cuestan menos que antes.
  = Maths is less difficult for me than before.

But in your sentence, menos simply means the difficulty is reduced in that situation.

Because it describes a habitual or general situation:

• when I study with my friend, maths is easier for me

Spanish often uses the present tense for repeated or usual actions, just like English does in sentences such as:

• I feel better when I sleep more.
• I learn faster when I practise every day.

So cuando estudio con mi amiga means when I study with my friend in a general sense, not just one single time.

Yes, depending on context, cuando + present subj/indicative? Let's be precise: in Spanish, after cuando, if you are talking about a habitual/general action, you use the present indicative, as here: cuando estudio.

But if you are referring to a specific future event, Spanish normally uses cuando + present subjunctive:

• Cuando estudie con mi amiga, entenderé mejor.
  = When I study with my friend, I’ll understand better.

So in your sentence, cuando estudio clearly sounds habitual, not a one-time future event.

Because in Spanish, possessive adjectives like mi, tu, su, nuestro, etc. normally replace the article.

So Spanish says:

• mi amiga = my friend
• mi libro = my book

Not:

• la mi amiga ❌

This is different from some older forms or some other languages, but in standard modern Spanish, mi amiga is correct.

No. It tells us the gender of the friend, not the speaker.

• mi amiga = my female friend
• mi amigo = my male friend

The word mi does not change depending on whether the speaker is male or female. A man and a woman would both say:

• mi amiga for my female friend
• mi amigo for my male friend

The written accent in matemáticas shows where the stress goes: ma-te-MA-ti-cas.

Without the accent, Spanish pronunciation rules would place the stress differently, so the accent mark is necessary.

This word is an example of an esdrújula word, and all such words carry a written accent in Spanish.

Yes. In Spain, las mates is a very common informal shortening of las matemáticas.

So:

• Las matemáticas me cuestan menos... = neutral/standard
• Las mates me cuestan menos... = more casual, everyday speech

Both are correct, but las matemáticas is better in formal writing or when you want a full standard version.

On its own, con mi amiga means with my friend, meaning together with her.

It does not normally mean at my friend’s house.

If you wanted to say at my friend’s house, you would usually say something like:

• en casa de mi amiga

So:

• estudio con mi amiga = I study with my friend
• estudio en casa de mi amiga = I study at my friend’s house

Yes, that is a very natural English translation.

Because costar in this context is about difficulty or effort, possible natural translations include:

• Maths is less difficult for me when I study with my friend.
• I find maths easier when I study with my friend.
• Maths comes easier to me when I study with my friend.
• I struggle less with maths when I study with my friend.

They all reflect the same basic idea, even if the wording changes.