The verbs let and make take a bare infinitive:
[1a] She let it go.
[1b] She made it go.
The infinitive marker (to) appears with the causative verbs cause and allow: [1c] She caused it to go.
[1d] She allowed it to go.
The aphorism Easy come, easy go is similar in structure to First come, first served, the longer version of which might be something like:
 The first person to come is the first person to be served => First come, first served.
The verb come is a bare infinitive (from to come); the past participle served is an adjective (from to be served). The longer version of Easy come, easy go might be something like,
 That which is easy to come is easy to go => Easy come, easy go.
The word easy is an adjective, the words come and go are bare infinitives.
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