El imán no me parecía interesante hasta que la profesora lo usó para explicar una fórmula de física.

Because parecía is the imperfect tense, which is often used for a background state, ongoing impression, or repeated feeling in the past.

Here, the idea is:

• Before that moment, the magnet didn’t seem interesting to me
• Then something changed: the teacher used it

So Spanish naturally uses:

• no me parecía interesante = it didn’t seem interesting to me / I didn’t find it interesting

If you said no me pareció interesante, it would sound more like a finished judgment at a specific moment:

• I didn’t find it interesting (at that particular time)

In this sentence, the speaker is describing a past perception that lasted up to a change, so parecía fits better.

Literally, me parecía means something like:

• it seemed to me
• or it was seeming to me

Breakdown:

• parecía = seemed
• me = to me

So:

• El imán no me parecía interesante
  = The magnet didn’t seem interesting to me

Spanish often uses this structure where English might simply say I didn’t find it interesting.

Me is the indirect object pronoun meaning to me.

With parecer, Spanish commonly says:

• me parece = it seems to me
• te parece = it seems to you
• le parece = it seems to him/her/you(formal)

So:

• El imán no me parecía interesante
  literally: The magnet didn’t seem interesting to me

Even though English often doesn’t say to me, Spanish usually does with parecer when you want to say how something seems to someone.

Lo is the direct object pronoun meaning it, and it refers to el imán.

So:

• la profesora lo usó = the teacher used it

Why lo?

• imán is masculine singular
• the masculine singular direct object pronoun is lo

Examples:

• el libro → lo
• el imán → lo
• la mesa → la

So lo usó means used it, referring back to the magnet.

Because el imán is the direct object of usar.

Ask the question:

• ¿Qué usó la profesora?
  El imán.

Since it answers what?, it is a direct object, so standard Spanish uses lo.

• lo usó = she used it

Le is normally for indirect objects:

• Le explicó la fórmula al alumno = She explained the formula to the student

In parts of Spain, you may hear le used for masculine people as a direct object (leísmo), but for a thing like imán, lo is the standard form.

Usó is in the preterite because it refers to a completed action:

• the teacher used it
• that happened as a specific event in the past

This contrasts nicely with parecía:

• parecía = ongoing background impression
• usó = the event that changed that impression

This is a very common Spanish pattern:

• imperfect for the situation/background
• preterite for the event that happened

So the sentence presents:

1. an ongoing past opinion
2. a completed event that changed things

Hasta que means until.

So:

• no me parecía interesante hasta que...
  = it didn’t seem interesting to me until...

It introduces the point when the situation changed.

In this sentence:

• for some time, the magnet seemed uninteresting
• then the teacher used it
• after that, the speaker’s view changed

Since the action after hasta que is a real completed past event, Spanish uses the indicative:

• hasta que la profesora lo usó...

After para to express purpose, Spanish uses the infinitive.

So:

• para explicar = to explain
• literally: in order to explain

That is why the sentence says:

• la profesora lo usó para explicar una fórmula
  = the teacher used it to explain a formula

You use this pattern all the time:

• Estudio para aprender. = I study to learn.
• Lo compró para usarlo mañana. = He/She bought it to use it tomorrow.

This means a physics formula.

Spanish often expresses this kind of idea with noun + de + noun:

• una fórmula de física = a formula of/from physics = a physics formula
• un libro de historia = a history book
• una clase de español = a Spanish class

So de física tells you what field the formula belongs to.

English often uses a noun directly before another noun (physics formula), but Spanish usually prefers de.

Spanish uses definite articles (el, la, los, las) more often than English.

Here, El imán means:

• The magnet

It may refer to:

• a specific magnet already known in the context, or
• the magnet being talked about in a story/example

In Spanish, starting a sentence with El imán... is very natural. English sometimes does the same, but Spanish generally uses articles more consistently.

Yes, Spanish could leave it out if the subject were already clear:

• ...hasta que lo usó para explicar...

That would still be grammatical, because usó already tells you it was he/she/you formal.

But la profesora is included to make the sentence clearer and more explicit:

• until the teacher used it...

Spanish often omits subject pronouns like ella, but nouns such as la profesora are used when you want to identify who did the action.

This is a very natural Spanish order.

Structure:

• no = not
• me = to me
• parecía = seemed
• interesante = interesting

So:

• no me parecía interesante
  literally: it didn’t seem interesting to me

Spanish object pronouns like me, te, lo, la usually go before a conjugated verb:

• me parecía
• lo usó

That is why you get:

• El imán no me parecía interesante...

Interesante is an adjective describing el imán.

In the structure parecer + adjective, the adjective tells you how something seems:

• parece fácil = it seems easy
• parecía raro = it seemed strange
• no me parecía interesante = it didn’t seem interesting to me

So interesante is the quality the magnet seemed to have, from the speaker’s point of view.

The accents show which syllable is stressed and sometimes distinguish the normal pronunciation pattern.

• imán → stress on the last syllable: i-MÁN
• fórmula → stress on the first syllable: FÓR-mu-la
• física → stress on the first syllable: FÍ-si-ca

These accents are required by Spanish spelling rules.

For a learner, the important thing is:

• always pronounce the stressed syllable clearly
• the written accent tells you where that stress goes

In standard Peninsular Spanish, parecía is pronounced roughly like:

• pa-re-THÍ-a in much of Spain

That is because the c before i is often pronounced like the th in thin in Spain.

Also:

• í shows the stress: pa-re-CÍ-a
• the í-a is pronounced in two syllables, not as one merged sound

So it is approximately:

• pa-re-THI-a
  with the stress on cí

In Latin American Spanish, you would usually hear:

• pa-re-SÍ-a

Yes. That is a very natural English translation.

Even though parecer literally means to seem, in context:

• El imán no me parecía interesante

can be translated as:

• The magnet didn’t seem interesting to me
• I didn’t find the magnet interesting

The exact English wording may change, but the Spanish structure still centers on how the magnet seemed to the speaker.