Modality

Necessity

Tha+feum+aig+subject+air+N or V

(1)

Tha	feum	aig	Calum	air	biadh
be.pres	need	at	Calum	on	food
Calum needs food

(2)

Tha	feum	aig	Calum	air	dol
be.pres	need	at	Calum	on	go.vn
Calum needs to go

Possibility

(1)

Faodaidh	Daidhidh	Iain	a	bhualadh.
may	David	John	Tran	strike.vn
'David may hit Ian.'

(2)

Dh'fhaodinn	ithe.
might.1s	eat.vn
'I might eat.'

(3)

Dh'fhaodadh	tu	ithe.
might	you(sg)	eat.vn
'You might eat.'

Obligation

(1)

Feumaidh	Daidhidh	am	balach	a	bhualadh.
must	David	the	boy	Tran	strike.vn
'David must strike the boy.'

Is copula introduces a modal complement

(1)

Is	còir	do	Iain	an	leabhar	a	cheannach.
cop.pres	right	to	John.dat	the.sm	book.sm	Tran	buy.vn
'John should buy the book.' (MacAulay 1992)

(2)

Is	dòcha	nach	do	cheannaich	Iain	an	leabhar.
cop.pres	possible	Neg.Comp	to	buy.pst	John	the.sm	book.sm
'It is possible that John didn't buy the book.' (MacAulay 1992)

External Links

Modal verb from Wikipedia