Moja baka se ne može popeti uz stepenice.

A word-by-word breakdown:

• moja = my
• baka = grandmother / grandma
• se = a reflexive particle, used with certain verbs
• ne može = cannot / is not able to
• popeti = to climb up
• uz = up / along / against
• stepenice = stairs

So the structure is basically:

My grandmother + cannot + climb up + the stairs.

Moja is the feminine singular nominative form of moj (my).

It has to agree with baka, because baka is:

• feminine
• singular
• nominative here, because it is the subject of the sentence

Some related forms are:

• moj brat = my brother
• moja sestra = my sister
• moje dijete = my child

So moja baka is the correct agreement.

In the present tense, Croatian verbs agree with the subject in person and number, but not in gender.

So:

• ja mogu = I can
• ti možeš = you can
• on/ona može = he/she can

Because baka is she and singular, the verb is može.

Gender does show up in some other forms, especially in the past tense:

• baka se mogla popeti = grandmother was able to climb up

Here mogla is feminine.

Se is there because the verb is popeti se.

Many Croatian verbs are used with se, and you often just have to learn them as a unit:

• smijati se = to laugh
• bojati se = to be afraid
• popeti se = to climb up

In this sentence, se does not literally mean herself in the English sense. It is simply part of the verb construction.

So you should learn:

• penjati se = to be climbing / to climb
• popeti se = to climb up successfully, to get up to the top

In Croatian, negation with verbs is usually formed by putting ne before the verb as a separate word:

• ne može = cannot
• ne znam = I do not know
• ne vidi = he/she does not see

So ne može is the normal spelling.

A few forms in Croatian are written together, but moći is not one of those in standard usage here. So ne može is correct.

After the modal verb moći (can / be able to), Croatian normally uses an infinitive:

• mogu doći = I can come
• može spavati = he/she can sleep
• ne može se popeti = he/she cannot climb up

So the pattern is:

moći + infinitive

Here:

• može = can
• popeti = to climb up

Together: može popeti se in basic logic, though in real Croatian the se is placed earlier: se ne može popeti.

This is mainly about aspect, which is very important in Croatian.

• penjati se is imperfective
  It focuses on the process: to be climbing, to climb in a general or repeated sense.
• popeti se is perfective
  It focuses on reaching the top or completing the action: to climb up successfully

So:

• Moja baka se ne može penjati uz stepenice
  = My grandmother cannot climb stairs / is unable to do stair-climbing in general.
• Moja baka se ne može popeti uz stepenice
  = My grandmother cannot get up the stairs.

In your sentence, popeti se sounds natural because it is about successfully getting up the stairs.

The preposition uz often means up, along, or against, and here it is used with the accusative.

So:

• uz stepenice = up the stairs

The noun stepenice is one of those nouns that usually appears in the plural when talking about a staircase or stairs.

So after uz, you get the accusative plural form:

• nominative plural: stepenice
• accusative plural: stepenice

These two forms happen to look the same here.

Like English stairs, Croatian normally uses a plural form here:

• stepenice = stairs
• stube = stairs, steps

A singular form such as stepenica usually means a step, not a staircase.

So:

• jedna stepenica = one step
• uz stepenice = up the stairs

That is why the plural is used in your sentence.

Croatian word order is more flexible than English word order, but not completely free.

The sentence:

Moja baka se ne može popeti uz stepenice.

is very natural and neutral.

A few points:

• se usually goes in an early position in the sentence
• ne goes directly before the verb it negates: ne može
• the subject moja baka can sometimes move for emphasis

For example, you may also hear:

• Moja baka ne može se popeti uz stepenice.

That is possible, but Moja baka se ne može popeti uz stepenice is generally the more natural neutral order.

Yes, absolutely.

• Baka se ne može popeti uz stepenice = Grandma can’t climb up the stairs.
• Moja baka se ne može popeti uz stepenice = My grandmother can’t climb up the stairs.

Adding moja makes it explicit that you mean my grandmother. Leaving it out can still sound natural if the context already makes that clear.

Croatian often leaves out words that are understood from context.

No. Another common word is stube.

So you may hear:

• uz stepenice
• uz stube

Both can mean up the stairs.

Very roughly:

• stepenice is widely understood and common
• stube is also very common, especially in some regional varieties

A learner should recognize both.

A more general statement would often use the imperfective verb:

• Moja baka se ne može penjati uz stepenice.

That suggests a general inability or difficulty with the activity.

Your original sentence with popeti se is more about a specific result:

• She can’t get up the stairs.

So the difference is:

• penjati se = the activity/process
• popeti se = successfully reaching the top

Both are correct, but they express slightly different meanings.