Lift je mali, a moja majka ide stepenicama.

Because mali has to agree with lift, which is a masculine singular noun.

In Serbian, adjectives agree with the noun in:

• gender
• number
• case

Here:

• lift = masculine singular
• so mali = masculine singular form of small

Compare:

• mali lift = a small elevator
• mala soba = a small room
• malo dete = a small child

So malo would be neuter, which does not match lift.

Je is the 3rd person singular present tense of biti = to be.

So:

• lift je mali = the elevator is small

This is the normal Serbian equivalent of the English verb is.

A quick comparison:

• ja sam = I am
• ti si = you are
• on/ona/ono je = he/she/it is

A often means and, while, or whereas, depending on context.

In this sentence, it links two ideas with a slight contrast:

• The elevator is small, and/while my mother takes the stairs.

It is not exactly the same as:

• i = plain and
• ali = but

So a is very natural here because it connects two related facts and lightly contrasts them.

Examples:

• Ja volim kafu, a ona voli čaj. = I like coffee, while she likes tea.
• On radi, a ja odmaram. = He is working, and I am resting / while I am resting.

Because Serbian normally uses a comma before coordinating conjunctions like a, especially when joining two full clauses.

Here the two clauses are:

• Lift je mali
• moja majka ide stepenicama

So the comma is standard punctuation.

Yes, you often could just say majka if the meaning is clear from context.

Serbian often omits possessive words when they are obvious, especially with family members. But moja majka is still completely normal and clear.

So both can work:

• Moja majka ide stepenicama.
• Majka ide stepenicama.

Using moja can:

• make it more explicit
• add a little emphasis
• sound more natural if the speaker wants to be clear

Ide is the 3rd person singular present tense of ići = to go.

So:

• moja majka ide = my mother goes / is going

Depending on context, Serbian present tense can mean:

• something happening now
• something habitual or usual

So this sentence could mean either:

• My mother is taking the stairs
  or
• My mother takes the stairs

The exact English translation depends on context.

Because after verbs like ići when you mean by means of or via, Serbian often uses the instrumental case.

So:

• stepenice = nominative/basic form
• stepenicama = instrumental plural

In this sentence:

• ide stepenicama = goes by the stairs / takes the stairs

This is similar to:

• ide autobusom = goes by bus
• ide kolima = goes by car
• ide liftom = goes by elevator

So stepenicama is the correct case here.

Usually, yes. In everyday Serbian, stepenice is commonly treated as a plural noun meaning stairs.

That is similar to English, where stairs is also usually plural.

So you will often see forms like:

• stepenice
• stepenicama
• na stepenicama

If you want to refer to a single step, Serbian usually uses stepenik.

So:

• stepenik = one step
• stepenice = stairs

Because with ići Serbian can use the instrumental case directly to show the means or route.

So:

• ići stepenicama = to go by the stairs / take the stairs
• ići liftom = to go by elevator
• ići autobusom = to go by bus

No preposition is needed.

If you used a preposition, the meaning would usually change. For example:

• na stepenicama = on the stairs
• uz stepenice = up the stairs
• niz stepenice = down the stairs

But stepenicama by itself here means the stairs are the way she is going.

Not specifically. Ide stepenicama means she is going by the stairs or using the stairs, but it does not say whether she is going up or down.

If you want to be specific, Serbian would usually say:

• ide uz stepenice = she is going up the stairs
• ide niz stepenice = she is going down the stairs

So the original sentence is more neutral: she is taking the stairs instead of using the elevator.

Yes. Serbian word order is more flexible than English word order, although some versions sound more natural than others.

The original sentence:

• Lift je mali, a moja majka ide stepenicama.

Possible variations include:

• Moja majka ide stepenicama, a lift je mali.
• A moja majka ide stepenicama, lift je mali.
  This is possible in some contexts, but less neutral.

The original order is probably the most natural if the speaker first mentions the elevator and then what the mother does.

Yes, lift is a very common word in Serbian.

So:

• lift = elevator

Learners sometimes notice that in some neighboring standards, especially Croatian, dizalo is common. But in Serbian, lift is completely standard and very common in everyday speech.

Yes, you could, but it is a bit less idiomatic in this exact context.

Compare:

• moja majka ide stepenicama = my mother takes the stairs
• moja majka koristi stepenice = my mother uses the stairs

Both are understandable, but ide stepenicama sounds more natural for ordinary movement.

Similarly:

• ide liftom = goes by elevator / takes the elevator sounds more natural than
• koristi lift = uses the elevator

unless you specifically want to emphasize using something.