Este horario no me encaja, porque la reunión termina justo cuando empieza mi clase de español.

In this sentence, encajar means to fit, to work, or to suit in the sense of time or arrangement.

So Este horario no me encaja means something like:

• This schedule/time doesn’t work for me
• This timing doesn’t fit for me

It is very common in Spain to use encajar for plans, schedules, appointments, and arrangements.

Examples:

• Mañana no me encaja. = Tomorrow doesn’t work for me.
• Esa hora me encaja bien. = That time works well for me.

The me shows for whom something works or does not work.

• Me encaja = it suits me / it works for me
• Te encaja = it suits you
• Le encaja = it suits him/her

So:

• Este horario no me encaja = This schedule does not work for me

It is similar to other Spanish structures where the person affected is shown with an indirect object pronoun:

• Me gusta = I like it
• Me viene bien = It suits me / it’s convenient for me
• No me encaja = It doesn’t fit for me

Because horario is a masculine noun:

• el horario
• este horario

Even though some Spanish words ending in -a are masculine, here the issue is simpler: horario ends in -o, and it is masculine.

So the demonstrative has to match:

• masculine singular: este
• feminine singular: esta

Examples:

• este horario
• esta reunión

Horario usually means schedule, timetable, or time arrangement.

In this sentence, it does not mean a single hour on the clock. It refers more broadly to the arrangement of times.

Compare:

• hora = hour / time
• horario = schedule / timetable

Examples:

• ¿Qué hora es? = What time is it?
• Mi horario de trabajo = My work schedule
• Este horario no me encaja = This schedule doesn’t work for me

Because here it means because, giving a reason.

• porque = because
• por qué = why

So:

• No puedo ir porque trabajo. = I can’t go because I’m working.
• ¿Por qué no puedes ir? = Why can’t you go?

In your sentence, the second part explains the reason the schedule does not work, so porque is correct.

Spanish often uses the present tense for:

• regular schedules
• fixed events
• planned events
• things happening as part of an arranged timetable

So la reunión termina and empieza mi clase can refer to scheduled future events, not only things happening right now.

English does this too sometimes:

• The meeting ends at 5
• My class starts at 5

So the Spanish present tense sounds very natural here.

Because Spanish usually leaves out subject pronouns when the verb form already makes the subject clear.

• termina already tells you the subject is él/ella/usted
• empieza does the same

And the nouns are explicitly there anyway:

• la reunión termina
• mi clase empieza

So adding pronouns would be unnecessary.

Justo cuando means just when or right when.

Here it emphasizes that the two events happen at the same moment:

• the meeting ends
• my Spanish class starts

So it suggests an exact clash in timing.

You could also hear:

• precisamente cuando
• justo al mismo tiempo que in some contexts

But justo cuando is very natural and common.

Yes. Comienza and empieza both mean starts/begins.

• empieza is very common and slightly more everyday
• comienza is also common, sometimes a little more formal or neutral depending on context

So these are both fine:

• La reunión termina justo cuando empieza mi clase.
• La reunión termina justo cuando comienza mi clase.

Yes. Acaba can often replace termina here.

• terminar = to finish/end
• acabar = to finish/end

So:

• La reunión termina justo cuando empieza mi clase
• La reunión acaba justo cuando empieza mi clase

Both are natural. In everyday Spanish, especially in speech, acabar is very common.

Because the normal and simplest way to say my Spanish class is:

• mi clase de español

Spanish usually puts possessives like mi, tu, su before the noun:

• mi clase
• tu reunión
• su horario

The longer form with mía is used more for emphasis, contrast, or special contexts:

• La clase es mía. = The class is mine.
• Una clase mía empieza a las seis. = One of my classes starts at six.

So in this sentence, mi clase de español is exactly what you would expect.

Because when naming a subject of study, Spanish normally uses de + subject with no article.

So:

• clase de español
• libro de historia
• profesor de matemáticas

Here español means the subject Spanish, not the Spanish language in a general, abstract way.

If you said del español, it would usually sound like you are talking about something specifically about the Spanish language itself, not simply a Spanish class.

Both are possible.

• Este horario no me encaja is straightforward and neutral.
• No me encaja este horario is also correct, but it can sound a bit more conversational or put slightly more focus on this schedule.

Spanish word order is more flexible than English, especially with pronouns and emphasis.

So:

• Este horario no me encaja = very standard
• No me encaja este horario = also natural

They are similar, but not always identical.

• me encaja = it fits, it matches my schedule/needs
• me viene bien = it suits me, it is convenient for me

In many situations, both work:

• Este horario no me encaja
• Este horario no me viene bien

But encajar often gives a stronger idea of things fitting together properly in a timetable or arrangement.

Because this sentence refers to a real, specific event seen as factual or scheduled.

Spanish uses the indicative after cuando when the speaker is referring to something presented as an actual event in a schedule or as a fact in context.

Here, the meeting and the class are treated as real planned events:

• la reunión termina
• mi clase empieza

If you were talking about an uncertain future event in a different kind of sentence, Spanish might use the subjunctive:

• Te llamo cuando termine la reunión. = I’ll call you when the meeting finishes.

So the choice depends on how the event is being presented.

Usually, yes, or at least it strongly suggests that.

Justo cuando implies that one event ends at the exact moment the other begins, or so close that there is effectively a conflict.

So the speaker is saying the schedule does not work because the meeting ends exactly when the class starts, leaving no gap.

Yes, very natural.

This sentence sounds like normal everyday Spanish from Spain, especially because of no me encaja, which is very common in Spain when talking about timing, plans, and arrangements.

A speaker in Spain would easily understand it and would likely say something very similar.