- "Sometimes, I drink tea." (front position)
- "I sometimes drink tea" (mid-position)
- "I drink tea sometimes" (end position)
Note that for mid position, the adverb precedes the main verb except for be, so we say e.g. "I am sometimes angry" rather than e.g. *"I sometimes am angry"
Some adverbs can go in all three positions (e.g. sometimes); others are more restricted. Even when different positions are possible, we may use one position more than the other(s). The rules around this are complex, and confusing, and it's probably best to split adverbs down into different types of adverbs and then look at the grammar for each type separpately.
Please don't confuse adverb with adverbial; adverbials cannot go in mid-position.