Learn Hardcore Swahili: Mwanafunzi huyu ni mvivu nyumbani, lakini mchapakazi shuleni. - This student is lazy at home, but hard-working at school.

Mwanafunzi means student / pupil (literally: a learner), and huyu means this (person, near me).
Together, mwanafunzi huyu is this student.
In Swahili, demonstratives like huyu usually come after the noun (mwanafunzi huyu) rather than before, unlike English this student. Both huyu mwanafunzi and mwanafunzi huyu are possible, but noun + demonstrative is more common and neutral.

What is the role of ni in mwanafunzi huyu ni mvivu?

Ni is the copula, equivalent to is / am / are when linking a subject with a noun or adjective.
Here, it links mwanafunzi huyu (this student) with mvivu (lazy): this student is lazy.
It doesn’t change with person or number: mimi ni, wewe ni, yeye ni, etc., all use ni.

Is mvivu an adjective or a noun, and does it agree with mwanafunzi in any way?

Mvivu is a noun-like word meaning a lazy person / lazy (one), but it often functions like an adjective in sentences.
It belongs to the m-/wa- noun class: mvivu (singular, person) → wavivu (plural, people).
It already carries the m- prefix that matches the m-/wa- class of mwanafunzi, so there is no extra agreement marker needed.

What is the difference between nyumba and nyumbani?

Nyumba means house / home as a plain noun.
Nyumbani is nyumba + -ni, where -ni marks a place / location.
So nyumbani means at home / in the house / home (as a place).
In this sentence, mvivu nyumbani means lazy at home.

What does lakini mean, and can it start a sentence like but in English?

Lakini means but / however.
It introduces a contrast between two ideas, just like English but.
You can put it between clauses (as in this sentence) or even start a new sentence with it:
Mwanafunzi huyu ni mvivu nyumbani. Lakini (yeye ni) mchapakazi shuleni.

Why is there no ni in the second part lakini mchapakazi shuleni?

It is understood but omitted.
The full, explicit form would be: lakini (yeye) ni mchapakazi shuleni – but (he/she) is hard-working at school.
In coordinated sentences, Swahili often leaves out repeated words like the subject (yeye) and even the copula ni when the meaning is clear from the first clause.

Why don’t we repeat mwanafunzi huyu in the second clause?

Repeating mwanafunzi huyu would be grammatically correct but unnecessary:
Mwanafunzi huyu ni mvivu nyumbani, lakini mwanafunzi huyu ni mchapakazi shuleni.
Swahili normally omits the repeated subject in the second clause when it is obviously the same.
So the shorter version is more natural and less repetitive.

What does mchapakazi mean, and how is it formed?

Mchapakazi means a hard-working person / a diligent worker.
It has the m- prefix for a person in the m-/wa- noun class, plus chapakazi, which carries the idea of working hard / putting in effort.
The plural is wachapakazi: hard-working people / diligent workers.

Why is it shuleni and not shule?

Shule means school as a plain noun.
Shuleni is shule + -ni, again using -ni to show location.
So shuleni means at school / in school.
In this sentence, mchapakazi shuleni means hard-working at school.

What exactly is the contrast being made by nyumbani vs shuleni?

The sentence contrasts two different locations, each with a different quality:

nyumbani – at home → the student is mvivu (lazy) there.
shuleni – at school → the student is mchapakazi (hard-working) there.
So the learner is lazy in the home environment but diligent in the school environment.

Could we say Mwanafunzi huyu ni mvivu nyumbani, lakini ni mchapakazi shuleni? Is that correct?

Yes, that is grammatically correct.
Adding ni in the second clause makes the structure more symmetrical: … ni mvivu … lakini ni mchapakazi ….
Native speakers often omit the second ni in everyday speech, but including it is fine and may feel slightly clearer or more formal to a learner.

Can we change the word order to Huyu mwanafunzi ni mvivu nyumbani…? Does it change the meaning?

Yes, Huyu mwanafunzi ni mvivu nyumbani, lakini mchapakazi shuleni is also correct.
Putting huyu before the noun can add a slight nuance of focus or emphasis on this particular student, but the basic meaning stays the same.
Both mwanafunzi huyu and huyu mwanafunzi are acceptable; mwanafunzi huyu is more typical neutral order.

How would this sentence look in the plural: These students are lazy at home, but hard-working at school?

You would change the nouns and describing words to their plural m-/wa- forms:

Wanafunzi hawa ni wavivu nyumbani, lakini wachapakazi shuleni.

mwanafunzi → wanafunzi (student → students)
huyu → hawa (this [person] → these [people, near])
mvivu → wavivu (lazy person → lazy people)
mchapakazi → wachapakazi (hard-working person → hard-working people)
Nyumbani and shuleni stay the same, as they are locative forms.