A conjunctive adverb can appear either at the beginning of a sentence or within it. When it starts a sentence, a comma should follow it.
Example: Nevertheless, he ate the ice cream.
When it’s within a sentence, how it’s punctuated depends on what surrounds it. If there’s an independent clause on each side, it needs to be punctuated as described in the previous post.
Example: Today is the first day of spring; however, I’m going to miss winter.
When a conjunctive adverb “interrupts” a sentence that contains only one independent clause, the adverb needs to be “encased” in commas.
Example: For breakfast, however, she prefers to eat oatmeal.
Note: If a group of words that begins with a capital letter and ends with a period lacks an independent clause, it’s a sentence fragment. (“Stop!” is a sentence because its subject, “you,” is implied, and its meaning is understood. It expresses a complete thought.)