On comprend pourquoi.
1	On	on	PRON	_	Gender=Masc|Number=Sing|Person=3|PronType=Ind	2	nsubj	_	wordform=on
2	comprend	comprendre	VERB	_	Mood=Ind|Number=Sing|Person=3|Tense=Pres|VerbForm=Fin	0	root	_	_
3	pourquoi	pourquoi	ADV	_	_	2	advmod	_	SpaceAfter=No
4	.	.	PUNCT	_	_	2	punct	_	_
