Jos ostoskori on liian raskas, otan ostoskärryn.

Yes. Jos means if and introduces a conditional clause (a condition).
In this sentence, the Jos-clause gives the condition: Jos ostoskori on liian raskas = If the shopping basket is too heavy.

In Finnish, it’s standard to use a comma between a subordinate clause and the main clause.
So Jos ostoskori on liian raskas, (subordinate clause) is separated from otan ostoskärryn. (main clause).

Otan is the 1st person singular present tense form of the verb ottaa = to take.
So otan means I take / I will take (Finnish present can cover both “I take” and “I’ll take,” depending on context).

Finnish often uses the present tense for near-future actions when the future meaning is clear from context (like a condition or a plan).
So otan ostoskärryn can naturally mean I’ll take a cart (then).

• ostoskori = a shopping basket (usually carried by hand)
• ostoskärry = a shopping cart/trolley (usually on wheels)

The sentence is basically: if the basket is too heavy, I’ll switch to a cart.

ostoskärryn is the object in a “total/complete” sense: you take one cart (a whole unit), so Finnish typically uses the genitive/accusative-looking -n form for the object in an affirmative sentence.

• otan ostoskärryn = I’ll take a (whole) cart (a specific choice/action completed)
• otan ostoskärryä (partitive) would suggest something less complete/ongoing or an indefinite “some cart,” which is not the normal idea here.

Because on is the verb to be, and Finnish uses a predicate adjective after to be in the nominative to describe the subject.

• ostoskori (subject) = nominative
• raskas (describing the subject) = nominative

So it literally matches: The basket is too heavy.

liian means too (as in “excessively”). It functions like an adverb modifying the adjective:

• liian raskas = too heavy

Finnish typically places liian directly before the adjective it modifies.

Yes, that’s common. niin can work like then in an if–then structure:

• Jos X, (niin) Y. = If X, (then) Y.

It’s optional here; leaving it out still sounds natural.

Yes. You can also say:

• Otan ostoskärryn, jos ostoskori on liian raskas.

Both are correct. Starting with the Jos-clause often emphasizes the condition first; starting with the main clause emphasizes the action first.