-lako

Where it goes. -(e)lako attaches to the conjugated verb, the auxiliary or a synthetic verb like dago. That verb comes at the end of the reason clause:

• Ez naiz etorri gaixorik nengoelako. (I did not come because I was ill.)
• Euskara ikasten ari naiz, Bilbon bizi naizelako. (I am learning Basque because I live in Bilbao.)

Common forms.

• naiz → naizelako, zara → zarelako, da → delako, gara → garelako, dira → direlako
• dut → dudalako, duzu → duzulako, du → duelako, dugu → dugulako, dute → dutelako
• nago → nagoelako, zaude → zaudelako, dago → dagoelako
• past: nintzen → nintzelako, zen → zelako, nengoen → nengoelako, zuen → zuelako

Notice three changes: a final -t becomes -d (dut → dudalako), a final -a becomes -e (da → delako), and the past -n drops (zen → zelako). This is the same ending as in the that clause -(e)la, with -ko added.

Answering Zergatik? A -lako clause can stand alone as an answer to zergatik: Zergatik goaz taxiz? — Berandu delako. Remember that zergatik is only a question word; it never means because.

Position. The reason can come after the main clause or before it. When it comes first, it is often the focus of the sentence: Zuk deitu didazulako etorri naiz (I have come because you called me).

Later you will meet other ways to give a reason: -(e)nez for a reason both people already know, and bait- in more formal writing.