olisi voinut

The perfect conditional (konditionaalin perfekti) is used to talk about the 'unreal past'—earlier situations considered possible or hypothetical, often unrealized. In this pattern, 'voinut' is the past participle of the verb 'voida' (to be able to), so the construction translates to 'could have' or 'would have been able to'. The perfect presents the possible event as earlier than a reference point. It often concerns a missed opportunity, but the form alone does not prove that the event never happened; context supplies that conclusion.

1. Formation
To form this, you need three parts:

• The auxiliary verb olla in the conditional mood (olisin, olisit, olisi...).
• The past participle of 'voida' (voinut or voineet).
• The main verb in the first infinitive.

The infinitive may be omitted when context supplies the action: Olisin voinut can mean 'I could have'. With a condition adverb, voida instead describes well-being: Olisin voinut huonosti (I would have felt unwell).

2. Negative Form
The negative is formed by placing the negative verb before the conditional auxiliary: en olisi voinut, et olisi voinut, ei olisi voinut (meaning 'I could not have' or 'I wouldn't have been able to', etc.). Note that while 'olisi' does not change for person in the negative, the participle must still agree in number with the subject in formal writing (e.g., emme olisi voineet).

3. Questions
To ask a question, attach the question clitic -ko/-kö to the auxiliary verb, not the participle. Example: Olisitko voinut tulla? (Could you have come?)

4. Passive Form
To say something 'could have been done' (impersonal), use the passive participle: olisi voitu. Example: Olisi voitu mennä kotiin. (One could have gone home.)

5. Agreement
In formal writing, the participle must agree in number with the subject:

• Singular: Hän olisi voinut auttaa. (He/she could have helped.)
• Plural: He olisivat voineet auttaa. (They could have helped.)

6. Spoken Language (Puhekieli)
In spoken Finnish, common shortened auxiliary forms include oisin, oisit, ois, and oisitte. Speakers vary in how many other standard or regional features they combine with this stem. For plural subjects, te and ne typically use the singular participle form (e.g., te oisitte voinu, ne ois voinu). For the first-person plural me, both me oltais voitu and me ois voitu are widely used in spoken Finnish. Note that oltais is a shortened form of the passive oltaisiin, not the active olisi.
Example: Ne ois voinu tulla. (They could've come.) instead of He olisivat voineet tulla.

Choose the Kind of Ability
Olisi voinut tehdä expresses general possibility or permission. Olisi osannut tehdä emphasizes learned skill or knowing how. Olisi pystynyt tekemään emphasizes having the capacity or circumstances to manage the action, whether physical or otherwise. Note the different following form: osata/voida + tehdä, but pystyä + tekemään.