En física, la profesora explica la gravedad con un imán y una pelota.

En física means in physics or, more naturally here, in physics class. Spanish often uses en + subject to give the context of what is happening. So the sentence is setting the scene first: this explanation is happening in the subject area of physics.

A more explicit version would be En la clase de física, but En física is shorter and very natural.

After a preposition like en, school subjects often appear without an article when you mean the subject in a general classroom sense.

• En física = in physics / in physics class
• La física = physics as a discipline or field

For example:

• En física estudiamos la energía. = In physics, we study energy.
• La física es interesante. = Physics is interesting.

So En física is the natural choice here.

The comma separates the introductory phrase En física from the main part of the sentence.

It works like an English opening phrase such as In physics, ...

In Spanish, this comma is often used when a phrase at the beginning sets the context, topic, or frame of the sentence. In a short sentence like this, you may sometimes see it omitted, but using it here is perfectly normal.

Because the sentence is talking about a specific teacher, not just the profession in general.

• la profesora = the teacher / the female teacher
• es profesora = she is a teacher

Spanish often uses the definite article when referring to a known person by role or title inside a sentence.

So:

• La profesora explica... = The teacher explains...
• Ella es profesora. = She is a teacher.

Yes. Profesora is the feminine form, and profesor is the masculine form.

• el profesor = the male teacher
• la profesora = the female teacher

In this sentence, the teacher is female, so la profesora is the correct form.

Explica is the third-person singular present tense of explicar.

It matches the subject la profesora, which is singular:

• yo explico
• tú explicas
• él/ella explica

So la profesora explica means the teacher explains.

Spanish uses the simple present very often for:

• habitual actions
• general statements
• actions happening in the present scene

So this can mean either the teacher explains or the teacher is explaining, depending on context.

It could, but it would change the feel slightly.

• La profesora explica... = the teacher explains / is explaining
• La profesora está explicando... = the teacher is in the middle of explaining right now

Spanish uses the simple present more broadly than English does, so explica is very natural here. You do not always need the progressive form.

Spanish often uses the definite article with abstract or general nouns where English does not.

So:

• la gravedad = gravity
• la historia = history
• el amor = love

In English, we usually say just gravity, but in Spanish la gravedad is normal and expected.

Here con means with or using.

It tells you what the teacher is using to explain gravity:

• con un imán = with a magnet
• con una pelota = with a ball

So the idea is that the teacher is using those objects as teaching aids or demonstration tools.

Because the nouns have different genders:

• imán is masculine, so it takes un
• pelota is feminine, so it takes una

Also, the indefinite articles are used because these are just objects being introduced, not specific ones already known from earlier context.

So:

• un imán = a magnet
• una pelota = a ball

There is no perfect rule that lets you predict the gender of every noun. You often have to learn it together with the word.

In this case:

• el imán
• un imán

The ending -án does not automatically tell you the gender. The safest habit is to learn nouns with their article:

• el imán
• la pelota
• la gravedad

That way, you learn the gender naturally.

They have accent marks because Spanish spelling shows where the stress falls.

• física is stressed on the first syllable: FÍ-si-ca
• imán is stressed on the last syllable: i-MÁN

The accent marks are needed because those stress patterns do not follow the default stress rules for those word endings.

By contrast, words like profesora, gravedad, and pelota do not need accent marks because their stress already follows normal Spanish spelling rules.

Sometimes, but pelota is the more natural choice here.

• pelota usually means a ball, especially a physical object like a sports ball or classroom prop
• bola can also mean ball, but it often has other meanings too, such as a sphere, lump, or rounded mass

In this sentence, una pelota sounds more natural for the object the teacher is using in a demonstration.

It can be changed, because Spanish word order is more flexible than English word order.

This version:

En física, la profesora explica la gravedad con un imán y una pelota.

puts the context first: in physics.

You could also say:

La profesora explica la gravedad con un imán y una pelota en física.

But that sounds a bit less natural in many contexts, because en física works nicely as an opening frame.

So the original order is chosen for clarity and emphasis, not because it is the only possible order.