Detail: Verbs can be modified by other verbs. Modifier verbs are called “helper” or “auxiliary” verbs, not adverbs. In “I will practice grammar.”, the verb “practice” is modified by the helper verb “will” which changes it from present tense to future tense.
If I’m understanding correctly: you have issues with the auxiliary verb concept (always? or more so nowadays?).
So in “I will eat later” you’re saying that “will” is the main verb and it is modified by eat?
Then in the example given in the article “I will practice grammar.” , you would now consider will the main verb modified by practice? Though I guess you never say what is the main verb there.