To say 'would', slot -isi- between the verb stem and the personal ending: ostan - ostaisin (I would buy), olen - olisin (I would be). In a hypothetical 'jos' sentence Finnish puts the conditional in BOTH halves, not just the 'would' half.
Examples
- Jos minulla olisi aikaa, matkustaisin Japaniin.
- If I had time, I would travel to Japan.
- Ostaisin talon, jos voittaisin lotossa.
- I would buy a house if I won the lottery.
- Haluaisin kupin kahvia.
- I'd like a cup of coffee.
The mistake to avoid
Leaving the 'jos' clause in the plain present ('jos minulla on aikaa, matkustaisin') - a hypothetical needs -isi- in both clauses.
Where this fits in
Suunnitelmat, unelmat ja ehdot, Finnish B1: Talk about the future, express wishes, and use conditionals.