No, under is not a verb. Both are prepositions, and in the example given previously, "bridge" is the object of the preposition in both cases. In most cases, they are interchangeable. I am under the bridge. I am underneath the bridge. In that case they are interchangeable - no difference in meaning. There are some cases like "Under water" or "underwater" where you would not use "Underneath," but even in that case you may find some locales where they may be used interchangeable. In most cases, they are interchangeable synonyms.

Correction: What is the difference between under & underneath? Please give me a short explanation and an example as to how I can understand these 2 words. Thanks. "Under" as a preposition can mean 1 - in a place which is directly below: Examples: He hid under the table. The coin rolled under the piano. 2 - less than: Examples: She is under thirty. He is under age. The table was sold for under $50. She ran a kilometre in under four minutes. "Underneath" as a preposition means under/beneath Examples: She wore a t- shirt underneath her jacket He was sheltering underneath a gum tree. As an adverb it also means under: Example: He was wearing a thin shirt with nothing underneath.

I go under the bridge. Then underneath the bridge is my car. Under is a verb while underneath is more descriptive. Does this help?