Learn Finnish: Mä ostais ton aseen, jos se ois laillinen. - I would buy that weapon, if it were legal.

It is derived from the noun laki (law). To form it, you take the weak stem of the word, lai-, and add the suffix -llinen. This suffix often acts like '-ful' or '-ly' in English, turning 'law' into 'lawful' or 'legal'.

Why did the 'k' from laki disappear when forming laillinen?

Adding the -llinen suffix creates a closed syllable, which triggers consonant gradation just like adding a genitive -n does. The strong 'k' in laki weakens and drops out entirely, leaving the stem lai- before the suffix is attached.

Why do the verbs look like ostais and ois instead of ostaisin and olisi?

This sentence is a great example of spoken Finnish. In colloquial speech, the first-person conditional ending -in frequently drops the final -n, turning ostaisin into ostais. We still know it means 'I would buy' because of the pronoun mä. Similarly, the verb olisi is very commonly shortened to ois.

Why is the object ton aseen in the genitive case here?

It is the total object of the sentence. Buying a weapon is an action intended to be completed and affects the whole object, so it takes the genitive-looking accusative case. Ton aseen is just the spoken Finnish version of tuon aseen (that weapon).