El trayecto en tranvía dura menos de lo que pensaba, así que llego tranquila a clase.

En + means of transport is the normal way to say how someone travels in Spanish.

• en tranvía = by tram
• en tren = by train
• en autobús = by bus

So El trayecto en tranvía means the tram journey or the journey by tram.

You may also hear ir en tranvía = to go by tram.

Not exactly.

• trayecto usually means the route, journey, or trip from one point to another, often focusing on the distance or time involved.
• viaje is broader and often means a trip in general, especially a longer one.

In this sentence, El trayecto en tranvía is the specific journey to class by tram, so trayecto sounds very natural.

Because the subject is El trayecto en tranvía, which is singular.

The verb durar means to last.

• El trayecto ... dura = The journey lasts ...
• If the subject were plural, it would be duran.

So the structure is:

• El trayecto = subject
• dura = verb

This is a very common Spanish pattern.

• menos de = less than
• lo que = what / than what
• pensaba = I thought

So:

• menos de lo que pensaba = less than I thought
• more literally: less than what I thought

This same structure appears with other comparisons too:

• más de lo que esperaba = more than I expected
• mejor de lo que imaginaba = better than I imagined

Because lo que refers to an idea or amount, not a specific noun.

In menos de lo que pensaba, lo que means something like what or the amount that.

You can think of it as:

• menos de [la cantidad/tiempo] que pensaba

Spanish often uses lo que when talking about an unspecified thing, amount, or idea.

Because pensaba here refers to a previous mental state or expectation, not a single completed thought.

• pensaba = I was thinking / I thought
• pensé = I thought (at one moment) / I decided

In this sentence, the speaker is comparing reality with what they had previously expected, so the imperfect sounds natural:

• dura menos de lo que pensaba = it takes less time than I thought

Using pensé would sound more like a specific one-time thought, which is less natural here.

Así que means so, therefore, or and so.

It introduces a result:

• El trayecto ... dura menos de lo que pensaba, así que llego tranquila a clase.
• The journey takes less time than I thought, so I arrive calm to class.

It is not the same as porque:

• porque = because (gives the reason)
• así que = so / therefore (gives the result)

Compare:

• Llego tranquila porque el trayecto dura menos de lo que pensaba.
• El trayecto dura menos de lo que pensaba, así que llego tranquila.

Because tranquila is describing the subject, not the verb.

Here, tranquila means the speaker arrives in a calm state. It agrees with the person who arrives.

• llego tranquila = I arrive calm / relaxed

This is very common in Spanish with verbs like llegar, salir, volver, quedarse, etc.

Compare:

• Llegó cansado = He arrived tired.
• Salió enfadada = She left angry.

Tranquilamente would mean calmly as an adverb, focusing more on the manner of the action, not the speaker’s state. In this sentence, tranquila is more natural.

Because the speaker is female, or is being presented with feminine agreement.

Spanish adjectives usually agree with the person they describe:

• llego tranquilo = if the speaker is male
• llego tranquila = if the speaker is female

So tranquila tells you something about the speaker.

Because ir/llegar a clase is a fixed, very common expression meaning to go/come to class or to class in a general sense.

• Voy a clase = I go to class
• Llego a clase = I arrive at class / I get to class

Using a la clase would usually sound more specific, as if you meant a particular classroom or a specific class session in a more concrete way.

In general Spanish often drops the article in set expressions with places or activities:

• ir a clase
• ir a casa
• estar en clase

Not really. Here tranquila means calm, relaxed, or not stressed.

It does not usually mean quietly in the sense of making little noise. For that idea, Spanish would more likely use something like:

• en silencio = silently
• sin hacer ruido = without making noise

So here the meaning is about the speaker’s emotional state, not the volume or noise level.

The main statement is in the present because it describes what is generally true now or what happens regularly:

• El trayecto ... dura = the journey takes / lasts
• llego tranquila = I arrive calm

Then pensaba refers to the earlier expectation.

So the sentence combines:

• present for the current reality
• imperfect for the previous expectation

That combination is very normal in Spanish.

Yes, but the structure would change slightly.

With durar, the journey is the thing that lasts:

• El trayecto dura menos de lo que pensaba.

With tardar, Spanish often focuses on how long something takes:

• El trayecto tarda menos de lo que pensaba is not the most standard.
• More natural would be:
  • Se tarda menos de lo que pensaba.
  • Tardo menos de lo que pensaba en llegar.

So durar is a very good choice here because the subject is el trayecto.