Learn Dutch: Die uitdrukking is lastig te verstaan. - That expression is hard to understand.

Both are correct! The structure you see here ([Subject] + is + [Adjective] + te + [Infinitive]) is a concise, natural way to express this idea. Notice how the object of the action (die uitdrukking) becomes the grammatical subject of the sentence. This mirrors the English structure 'That expression is hard to understand.'

Since this lesson covers splitting separable verbs with te (like op te lossen), why doesn't verstaan split?

Because verstaan is an inseparable verb. Verbs that begin with unstressed prefixes like be-, er-, ge-, her-, ont-, and ver- never split. Because it cannot be separated, the te is simply placed directly in front of the entire word rather than sneaking inside it.

Both verstaan and begrijpen translate to 'to understand'. Does it matter which one I use here?

Yes, there is a big difference! Verstaan refers to auditory understanding—being able to physically hear and make out the words. Begrijpen refers to comprehending the meaning. If an expression is lastig te verstaan, it means the speaker is mumbling, speaking too fast, or there is background noise. If the concept or meaning itself is confusing, you would say it is lastig te begrijpen.