Learn Hardcore Finnish: Tämä tie on helpompi ajaa kuin edellinen. - This road is easier to drive than the previous one.

On is the 3rd person singular present of olla (“to be”). Here it acts as the copula linking the subject Tämä tie (“this road”) with the predicate helpompi ajaa kuin edellinen (“easier to drive than the previous one”).

Why is helpompi used instead of helppo?

Because you are making a comparison between two roads. In Finnish you form the comparative of adjectives by adding -mpi to the stem. So helppo (“easy”) → helpo- + -mpi → helpompi (“easier”).

Why does helpompi have only one p, while helppo has pp?

This is due to consonant gradation. The strong-grade pp in helppo becomes weak-grade p when you attach the comparative suffix -mpi, yielding helpompi.

How does the structure helpompi ajaa work grammatically?

Finnish often combines a comparative adjective with a bare infinitive to say something is “easier/difficult/etc. to do”. The pattern is:
adjective (comparative) + infinitive
Here helpompi is the comparative of helppo and ajaa is the 1st infinitive (“to drive”), so together they mean “easier to drive”.

Why is there no explicit subject for the infinitive ajaa?

The infinitive’s subject is understood to be the same as the main clause’s subject (this road, in a sense “it”). Finnish lets you omit that when it’s obvious, similar to English “This road is easier to drive.”

What does kuin edellinen mean and why is edellinen in the nominative case?

KuIn is the conjunction “than” in comparative structures. Edellinen is a nominalized adjective meaning “the previous one.” In Finnish comparisons introduced by kuin, the thing you compare to typically stays in nominative if it stands alone. Full form would be edellinen tie (“the previous road”), both words in nominative.

Could you include the noun tie after edellinen, and if so, how?

Yes. You can say:
Tämä tie on helpompi ajaa kuin edellinen tie.
Here both edellinen and tie remain in nominative singular. Omitting tie is common when it’s clear you mean “road.”

How flexible is the word order in this sentence?

Finnish is fairly flexible, but the neutral word order here is Subject – Verb – Predicate:
Tämä tie (S) on (V) helpompi ajaa kuin edellinen (Pred).
You can shuffle elements for emphasis, but this order is the most straightforward.

Is there a more formal or nominalized way to express the same idea?

Yes, you can nominalize the verb:
Tämän tien ajaminen on helpompaa kuin edellisen.
Here Tämän tien is genitive (“of this road”), ajaminen is the noun “driving,” and helpompaa is the partitive comparative of helppo. Edellisen is in partitive to match the comparison. This version is correct but more wordy.