Some infinitive verbs do not need the preposition "à" to mean "To (something)" since they are already constructed that way.
There is a long, long list of infinitive verbs that do need "à" before the infinitive. The "à + infinitve" rule is usually used when the infinitive shows tendency, aim or yearning, for a few examples:
Je m'attends à gagner. (I expect to win.)
Elle tarde à arriver. (She is late in arriving.)
Il commence à comprendre. (He is beginning to understand.)
It's just a matter of practicing, there are too many verbs to list 'em all here. I hope I could have helped in some way, i'm still a student after all.
Duo's hints of "enough" and "quite" may be true but they don't tell the whole picture. These words mean very different things and though represented by the single word "assez", they require two different words in English. So care must be taken to choose the correct one. Using "quite" in this sentence just doesn't work.
Generally, that would be wrong because it's implied that this sentence is referring to the word 'food', and in english 'food' is singular. As in, "There is always enough (food) to eat." However, in certain contexts where you have referred to a specific type of food earlier in a conversation, your sentence would be acceptable. "Are there enough apples to eat?" "We have a whole orchard! There are always enough to eat!"
Exceptions would be foods like bread or fish or beef that are still treated as singular "There is enough beef" not "There are enough beef"
I don't know the grammatical rule, but in English the adverbs "always" and "never" are usually placed immediately before the verb, except with "is", in which case they are placed immediately after. Examples: - He always eats bread. - He is always late. The "always" must go in the position shown above, and anywhere else would be incorrect, unless you're writing poetry.