Cada vez que estudiamos física, mi amiga escribe la fórmula en la pizarra.

Cada vez que means every time or whenever.

It introduces a repeated situation:

• Cada vez que estudiamos física... = Every time we study physics...

A very common pattern is:

• Cada vez que + verb, main clause

For example:

• Cada vez que llueve, me quedo en casa.
  Every time it rains, I stay at home.

So in your sentence, it sets up a habitual action: whenever the first thing happens, the second thing happens too.

Because the sentence describes a habitual or repeated action, not a single event happening right now.

• Cada vez que estudiamos física... = Every time we study physics...

Spanish often uses the present tense for things that happen regularly:

• Cuando trabajo, escucho música.
  When I work, I listen to music.

Here, estudiamos is not mainly we are studying right now, but we study / whenever we study.

It can mean either, depending on context.

• estudiamos = we study
• sometimes also we are studying

But in this sentence, because of cada vez que, the natural meaning is we study in a repeated sense:

• Cada vez que estudiamos física...
  Every time we study physics...

If you wanted to stress an action happening right now, Spanish would more often use:

• estamos estudiando

But that would not fit as naturally in this sentence about repeated occasions.

When talking about school subjects, Spanish often does not use the article.

So:

• estudiamos física = we study physics
• estudio matemáticas = I study maths
• tengo historia mañana = I have history tomorrow

Using la can sometimes sound more specific, more abstract, or less natural in a basic school-subject context.

So estudiamos física is the normal choice here.

They have written accents because of Spanish stress rules.

• física
• fórmula

Both words are stressed on the first syllable:

• FÍ-si-ca
• FÓR-mu-la

Without the accent mark, Spanish spelling rules would suggest a different stress pattern. The accent shows the correct pronunciation.

This is a spelling issue, not something that changes the basic meaning.

Mi amiga means my friend.

• mi = my
• amiga = female friend

Spanish often uses possessives like English does when you want to identify whose person or thing it is.

So:

• mi amiga escribe... = my friend writes...

If you said just amiga, it would usually need some other structure and would not mean exactly the same thing in this sentence.

Because amiga is the feminine form.

• amigo = male friend
• amiga = female friend

Since the sentence says my friend and refers to a female person, Spanish uses amiga.

Also notice:

• mi amigo
• mi amiga

The possessive mi does not change for masculine/feminine in the singular.

Because this sentence talks about what usually happens, not what is happening at this exact moment.

• escribe = writes
• está escribiendo = is writing

With cada vez que, Spanish normally uses the simple present for repeated actions:

• Cada vez que cocino, mi hermano canta.
• Every time I cook, my brother sings.

So:

• mi amiga escribe la fórmula en la pizarra
  = my friend writes the formula on the board

That sounds natural for a repeated classroom habit.

Spanish often uses the definite article el / la / los / las more often than English.

Here, la fórmula suggests a formula that is understood in context — for example, the one they are working on in class.

So:

• escribe la fórmula = she writes the formula

In English, we might sometimes say writes the formula or writes a formula, depending on the situation. In Spanish, la fórmula sounds natural if the formula is identifiable from the context.

Because en means on / in, while a usually means to.

• escribir en la pizarra = to write on the board
• ir a la pizarra = to go to the board

So:

• mi amiga escribe la fórmula en la pizarra
  = my friend writes the formula on the board

If you used a la pizarra, it would suggest movement toward the board, not the location where the writing appears.

Yes, pizarra is very common in Spain for a classroom board.

Depending on the region or country, you may also hear:

• pizarrón
• tablero
• encerado

But for Spain, pizarra is a very normal and useful word to learn.

Because Spanish often omits subject pronouns when the verb ending already shows who the subject is.

• estudiamos clearly shows we
• escribe clearly shows he/she/it

So Spanish does not need to say:

• Cada vez que nosotros estudiamos física, ella escribe...

That would be grammatical, but usually unnecessary unless you want emphasis or contrast.

Natural Spanish often sounds more like:

• Cada vez que estudiamos física, mi amiga escribe...

The word order is natural and straightforward:

• Cada vez que estudiamos física, = time clause first
• mi amiga escribe la fórmula en la pizarra = main clause

Spanish often places the time or condition clause first, especially with expressions like cada vez que, cuando, si, etc.

The main clause then follows in a normal order:

• subject: mi amiga
• verb: escribe
• object: la fórmula
• place: en la pizarra

This order is clear and very typical.

The comma separates the introductory clause from the main clause:

• Cada vez que estudiamos física,
• mi amiga escribe la fórmula en la pizarra.

This is common when a subordinate clause comes first. It helps readability.

Even if punctuation conventions can vary slightly, this comma is completely normal and helpful here.