Kama baba asingekuwa amebadilisha gia mapema, gari lisingekuwa limepanda mlima vizuri.

It is a past unreal / counterfactual conditional.

The idea is:

• If father had not changed gear early...
• ...the car would not have climbed the hill well.

So the sentence talks about an imagined past situation that did not actually happen that way. It strongly suggests that:

• baba did change gear early
• and because of that, the car managed the hill well

You can analyze asingekuwa amebadilisha as:

• a- = he
• -si- = negative
• -nge- = conditional, often giving the idea of would
• -kuwa = be
• amebadilisha = has changed / changed

So literally, it is something like:

• he would not be having changed

But in natural English in this kind of sentence, it means:

• he had not changed

This is a very common Swahili way to express a counterfactual past.

Swahili often uses:

• ngekuwa / singekuwa + perfect form

to express would have... or had... in unreal past situations.

So:

• asingekuwa amebadilisha = had not changed
• lisingekuwa limepanda = would not have climbed

This structure makes the meaning clearly about a completed past action and its imagined result.

A shorter structure is sometimes possible, but this version is especially clear and natural for a past counterfactual.

Here is the breakdown:

• li- = subject marker for gari
• -si- = negative
• -nge- = conditional
• -kuwa = be
• limepanda = it has climbed / it climbed

So literally it is something like:

• it would not be having climbed

But the natural English meaning is:

• it would not have climbed

or in this sentence:

• the car would not have gone up the hill well

Because gari belongs to a noun class that takes li- in the singular.

So:

• gari = car
• subject marker for singular gari = li-
• plural magari would take ya-

That means:

• gari limepanda = the car has climbed
• magari yamepanda = the cars have climbed

This is a noun class agreement issue, and it is very important in Swahili grammar.

Mapema means early.

So:

• kubadilisha gia mapema = to change gear early

Its position is normal. In Swahili, adverbs like mapema often come after the object or after the main verb phrase.

You may think of the order as:

• changed gear early

which is very similar to English.

Because in Swahili, kupanda is commonly used directly with things like:

• mlima = hill/mountain
• ngazi = stairs
• mti = tree

So:

• kupanda mlima = to climb a hill / go up a hill

Swahili does not need a word like up here. The meaning is already contained in kupanda.

So the phrase is completely normal:

• gari limepanda mlima = the car has climbed/gone up the hill

Because Swahili does not normally use articles like English a, an, and the.

So:

• baba can mean father, the father, or dad, depending on context
• gari can mean a car or the car
• mlima can mean a hill or the hill

The exact meaning comes from the situation, not from separate article words.

Vizuri means well.

So:

• limepanda mlima vizuri = it climbed the hill well
• lisingekuwa limepanda mlima vizuri = it would not have climbed the hill well

It describes how the car climbed the hill.
Without vizuri, the sentence would simply say the car would not have climbed the hill.
With vizuri, it specifically says the car would not have climbed it well / properly / successfully.

Yes.

The original order:

• Kama baba asingekuwa amebadilisha gia mapema, gari lisingekuwa limepanda mlima vizuri.

You can also put the result first:

• Gari lisingekuwa limepanda mlima vizuri kama baba asingekuwa amebadilisha gia mapema.

Both are grammatical.
Putting the kama clause first is very common because it sets up the condition before giving the result.

Yes, you may also hear a shorter version such as:

• Kama baba asingebadilisha gia mapema, gari lisingepanda mlima vizuri.

This is also understandable and natural.

The difference is that the original sentence:

• asingekuwa amebadilisha
• lisingekuwa limepanda

makes the past counterfactual meaning especially explicit, a bit like English had changed / would have climbed.

So the sentence you were given is slightly fuller and more precise in showing that the action is imagined as already completed in the past.