Learn Hardcore Swahili: Kama mngekuwa makini zaidi jana, msingepoteza simu yenu. - If you all had been more careful yesterday, you would not have lost your phone.

Notice that both verbs have -nge- (conditional), and the second one has si to make it negative. That is very typical for this type of unreal past condition.

Why do we have both kama and the -nge- form? Can I omit kama?

Kama means if (it can also mean like/as in other contexts).
The marker -nge- already expresses “would / would have” (a hypothetical situation).

So this sentence is literally:

Kama mngekuwa... – If you would have been...
msingepoteza... – you would not have lost...

Because -nge- already shows the conditional idea, kama is actually optional here.

You could also say:

Mngekuwa makini zaidi jana, msingepoteza simu yenu.

This still means exactly If you had been more careful yesterday, you wouldn’t have lost your phone.

Guideline:

With or without kama is both correct.
Using kama often feels a bit closer to English if, and is very common in speech.
The -nge- is the crucial part for this unreal/hypothetical meaning.

Why is it makini zaidi and not zaidi makini?

In Swahili, the typical order is:

Noun
Adjective
zaidi (more/most), which behaves like an adverb

So:

makini = careful
makini zaidi = more careful / especially careful

Examples:

mtu makini zaidi – a more careful person
rafiki mkarimu zaidi – a more generous friend

Zaidi makini is not the usual order; it sounds wrong or at least very unnatural in this kind of phrase. Put zaidi after the adjective.

What does yenu refer to in simu yenu, and how does it agree with simu?

Yenu means your (plural).

Swahili possessive adjectives agree with the noun class of the thing possessed.
Simu (phone) is a class 9/10 noun (often called the N-class), and for this class the possessive stem -enu takes the prefix y-:

simu yangu – my phone
simu yako – your (sg) phone
simu yake – his/her phone
simu yetu – our phone
simu yenu – your (pl) phone
simu yao – their phone

So in the sentence:

simu yenu = your (plural) phone

The y- in yenu is there because simu is in the N-class (class 9/10).

How would the sentence change if I’m talking to just one person instead of a group?

You would switch all the plural “you” forms to singular “you”:

Plural:
Kama mngekuwa makini zaidi jana, msingepoteza simu yenu.
If you (all) had been more careful yesterday, you (all) wouldn’t have lost your phone.
Singular:
Kama ungekuwa makini zaidi jana, usingepoteza simu yako.
If you (one person) had been more careful yesterday, you wouldn’t have lost your phone.

Changes:

mngekuwa → ungekuwa (you pl → you sg)
msingepoteza → usingepoteza (you pl → you sg, negative conditional)
simu yenu → simu yako (your pl → your sg)

Can I swap the order of the two clauses, starting with msingepoteza simu yenu?

Yes. Just like in English, you can put the result clause first:

Msingepoteza simu yenu, kama mngekuwa makini zaidi jana.
You wouldn’t have lost your phone, if you had been more careful yesterday.

or (more commonly, without a comma in everyday writing):

Msingepoteza simu yenu kama mngekuwa makini zaidi jana.

The meaning does not change. It’s just a different stylistic choice, exactly like English:

If you had been more careful, you wouldn’t have lost your phone.
You wouldn’t have lost your phone if you had been more careful.

Why is there no separate word for would or have like in English “would have lost”?

Swahili usually expresses things like would, have, had, will, etc. with markers inside the verb, not with separate helper words.

In msingepoteza:

-nge- does the job of English would / would have
There is no separate word for have; Swahili does not need an extra auxiliary here.
The past time is understood from context and from jana (yesterday).

So:

msingepoteza alone can cover the idea would not have lost.
mngekuwa covers would have been / had been.

Swahili packs a lot of what English uses auxiliaries for (would, have, had, etc.) into the verb form itself via prefixes and infixes like -nge-, plus contextual time words such as jana.