Q. I recommended to an author that he should use the word similar (no ly) when it comes before the word to (similar to, rather than similarly to), and should use the word similarly (with an ly) when followed by a comma. I cannot find a rule to cite. Am I correct? Thanks for your help.
Example 1: Similar to the credit crisis in the 1980s . . .
Example 2: Similarly, the recent financial crisis . . .
A. While your examples are correct, oversimplifications like this can go terribly wrong when applied universally or mechanically. Actual usage depends on syntax and context. Similar may be followed by a comma, and similarly to may be perfectly grammatical. For example:
The train runs clockwise, similarly to a clock.
Similar, but not the same, are trains that run counterclockwise.
In fact, no rule is needed, because the uses of the adjective similar and the adverb similarly are dictated by their definitions and parts of speech. When editing, if you need to change one and you feel that an explanation is needed, you can simply say “Adverb needed here” or “Syntax requires adjective.”