Li uzos lambastonon, krom se li bone iros.

Krom se means unless or more literally except if.

So:

• Li uzos lambastonon = He will use a crutch
• krom se li bone iros = unless he walks well / unless he will be able to walk well

It introduces an exception to what was just said.

Because -os is the Esperanto future tense ending.

• uzas = uses / is using
• uzis = used
• uzos = will use

So Li uzos means He will use.

The -n marks the direct object.

In this sentence, the thing being used is the crutch, so lambastonon is the object of uzos.

• li = the subject, the person doing the action
• uzos = will use
• lambastonon = the thing he will use

This is the normal Esperanto accusative ending.

Lambastono means crutch.

It is made from:

• lam- = related to limping or lameness
• bastono = stick

So the word literally suggests a stick for someone who limps, which is why it means crutch.

Because bone is an adverb, and it describes the verb iros.

• bona = good, an adjective used with nouns
• bone = well, an adverb used with verbs

Here the idea is he will walk well or he will go well, so Esperanto uses bone.

Compare:

• bona viro = a good man
• li bone iras = he walks/goes well

Yes, iri usually means to go, but depending on context it can also mean to walk or to move about.

In this sentence, because it talks about using a crutch, li bone iros is naturally understood as something like:

• he will walk well
• he will get around well
• he will be able to move well

So this is a context-based meaning, and that is very normal in Esperanto.

Because krom se li bone iros is its own clause, and that clause needs a subject.

English does the same thing:

• He will use a crutch, unless he walks well.

You still say he in the second part. Esperanto works the same way here.

Yes. That would also be grammatical.

Esperanto word order is fairly flexible, and both of these are possible:

• li bone iros
• li iros bone

Both mean roughly the same thing. Putting bone before the verb can give it a slight emphasis, but in many contexts the difference is very small.

Esperanto uses la only when something is definite and specifically identified as the thing.

Here, lambastonon is indefinite in the sense of a crutch, not necessarily the crutch. So no article is needed.

• Li uzos lambastonon = He will use a crutch
• Li uzos la lambastonon = He will use the crutch

Since Esperanto has no separate word for a/an, the bare noun often has that meaning.

The comma separates the main clause from the conditional clause.

• Li uzos lambastonon = main statement
• krom se li bone iros = exception/condition

In Esperanto, commas are often used a bit more regularly to separate clauses than in English, so this punctuation is completely normal.