"I want to improve the color."
Translation:Voglio migliorare il colore.
Is there a rule as to when you're supposed to include "a" (meaning "to") in a phrase like this one, as in "Voglio a migliorare il colore"?
Volere is a modal verb (they are four, all together) and never takes a preposition, being followed immediately by the infinitive of the second verb:
(Io) voglio migliorare. = I want to improve.
(Tu) vuoi andare. = You want to go.
(Lui) sa leggere. = He can/is able to read.
(Lei) può uscire. = She can go out.
A small group of other verbs are followed directly by the infinitive of the second verb, but are not modals. The most important ones are:
amare = to love
desiderare = to wish, would like
preferire = to prefer
gradire = to like, enjoy (formal)
osare = to dare
fare = to make
lasciare = to let
(Io) amo dipingere = I love painting
(Lei) desidera riposare = She would like to rest.
(Noi) preferiamo attendere. = We prefer to wait.
(Lui) lascia venire (qualcuno). = He lets (someone) come.
(Loro) fanno fermare (qualcosa/qualcuno) = They make (someone/something) stop.
The last two examples feature verbs fare and lasciare, whose construction is identical to that of modal verbs, but is known as causative construction (the difference goes beyond the purpose of this comment).
A larger number of verbs can be followed by the infinitive of a second verb only if a preposition acts as a link between the two (aspectual construction). They are ordinary verbs, which are said to be aspectual only when they are used for this construction.
The prepositions that link an aspectual verb to the infinitive of the following one are a and di.
Which preposition is required depends on the aspectual verb, and should be learned.
finire di lavorare = to finish working
- (Loro) finiscono di lavorare alle otto. = They finish working at eight o'clock.
cominciare a dipingere = to start painting
- (Lei) ha cominciato a dipingere. = She (has) started painting.
fingere di sapere = to pretend to know
- (Lui) fingeva di sapere la risposta. = He pretended to know the answer.
riuscire a comprare = to succeed/be able to buy
- (Noi) siamo riusciti a comprare la casa. = We were able to buy the house.
smettere di fumare = to stop smoking
- (Tu) hai smesso di fumare? = Have you stopped smoking?
stentare a comprendere = to have difficulty in understanding
- (Io) stento a comprendere le regole. = I have difficulty in understanding the rules.
tentare di venire = to try to come
- (Voi) tenterete di venire? = Will you try to come?
More about aspectual verbs can be found in this discussion: