A simple verb is one word with no helper in front of it: She runs. A verb phrase adds an auxiliary before the main verb: She is running. To identify one, check whether an auxiliary stands before the verb in the sentence. If no helper appears, the verb is simple. Same word, different roles: is is simple in She is kind, auxiliary in She is running.
Recognition examples
Statement with one main verb. I work. The verb carries the meaning by itself, with no helper before it.
Command with one main verb. Pass. The single verb gives the order directly, no auxiliary in front.
Negative command with helper do. Don't come near me. The word before the main verb is the auxiliary do, so this is not a simple verb but a verb phrase with an auxiliary do. The negation boundary section below explains this case in more detail.
Past form standing alone. The glass spilled. No helper is present, so spilled is a simple verb.
Past form with an auxiliary. The glass was spilled. The helper was sits before the verb, so this is a verb phrase, not a simple verb.
The negation boundary
One verb per clause is not the whole test. A helper word before the main verb makes the verb phrase, whether or not negation appears. She will run. uses will as an auxiliary and is not a simple verb, even though nothing is negated.
Negation is where this boundary shows up most clearly. An affirmative command like Pass. is a simple verb, but its negative form Don't come near me. needs the auxiliary do, so it counts as a verb phrase.
The same shift appears in statements. I work. stays simple, while I don't work. adds do and stops being simple. Negation itself does not change the verb's meaning, only its classification.
The test stays simple: check for a helper word before the main verb. Do, will, and be in was spilled all give the same verdict: verb phrase, not simple verb.