Does sakna has special deal with verbs
so we use only the root form?
Because when we use chahana the verb come in the infinitive form
आमीर पढ़ना चाहता है
Yes. It's because 'sakna' is an auxiliary verb and is only used along with other verbs.
'Chahna' is a regular verb and the other verb (in the infinitive form) takes the place of an object.
It's a similar deal as with 'can' and 'want' in English.