-(e)na da

How to build it. Put the element you want to stress right before a form of izan, then add a relative clause with the article:

• Ane da + etorri den + -a → Ane da etorri dena (It is Ane who came)
• Hau da + nahi dudan + -a → Hau da nahi dudana (This is what I want)

Always without -k. The stressed element is the subject of izan, so it keeps the plain form even if it did the action: Anek hori esan du becomes Ane da hori esan duena.

One or many. The relative takes -a for one person or thing and -ak for several, and izan agrees: Haiek dira etorri direnak (It is they who came). With ni or zu, the verb inside often agrees with them too: Ni naiz hori esan dudana (It is me who said that). You will also hear the third-person form: Ni naiz hori esan duena.

Tense. For the past, use izan zen or zen with a past relative: Mikel izan zen leihoa hautsi zuena (It was Mikel who broke the window).

Both orders. You can also start with the relative: Nahi dudana hau da (What I want is this). In both orders the stressed word stands right before izan, in the focus position.

When to use it. Everyday Basque often stresses a word just by putting it before the verb: Anek ekarri du ogia (It is Ane who has brought the bread). The cleft is stronger. It is useful for correcting someone, for questions like Zer da behar duzuna? (What is it that you need?), and for long ideas that need a clear frame.