To talk about a past that didn't happen — if I had known…, I would have come — take the perfect tense from unit 13 and put olla in the conditional:

Perfect — it happened	Past conditional — it didn't
olen tullut — I have come	olisin tullut — I would have come
olet tullut	olisit tullut
hän on tullut	hän olisi tullut
olemme tulleet	olisimme tulleet
olette tulleet	olisitte tulleet
he ovat tulleet	he olisivat tulleet

As in the perfect, the participle is -nut / -nyt for one person and -neet for more.

The if-sentence takes it in both halves:

Jos olisin tiennyt, olisin tullut. — If I had known, I would have come.