Learn Dutch: Proef dit hapje eens. - Just have a taste of this snack.

It softens the command. Without eens, Proef dit hapje sounds like a direct, bossy order: "Taste this snack!" By adding eens, it turns into a friendly suggestion, similar to adding "just" or "why don't you" in English.

Is "eens" pronounced exactly as it is spelled here?

When used as a modal particle to soften a sentence, native speakers almost always shorten eens to 'ns or 'es in spoken Dutch. You will usually hear something that sounds like "Proef dit hapje es."

Why is there no subject like "je" or "jij" in this sentence?

Because this is an imperative (a command or request). In Dutch, just like in English, you give a direct command by starting with the verb and dropping the subject. We just use the stem of the verb proeven (to taste), which is proef.

Why do we use "dit" instead of "deze" before "hapje"?

Hapje (snack or little bite) is a diminutive, which you can tell by the -je ending. In Dutch, all diminutives are het words. Because it is a het word, we have to use dit (this) instead of deze.