What is the infinitival "to" key function that distinguishes it from other morphemes like -ing or -en ?

Question

What is the infinitival "to"  key function  that distinguishes it from other morphemes like  -ing  or  -en ?

Answer

As a particle; it serves to oppose all infinitives to all non-infinitives.

Answer

As a word-morpheme and group-morpheme; it can represent the whole analytical word.

Answer

As a grammatical morpheme; it forms tense opposemes, similar to the suffix  -ed .

Answer

As a group-morpheme; it cannot be separated from the rest of the analytical word by other words.

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