Similar is an adjective and describes a thing: "a similar problem". Similarly is an adverb and describes how something is done, or links two sentences: "she answered similarly". If the word is sitting next to a noun, you want similar; if it is modifying a verb or introducing a comparison, you want similarly.
The core difference
These are the same word in two grammatical jobs, and the -ly ending is the signal. Similar modifies nouns. Similarly modifies verbs, adjectives and whole clauses.
- We had a similar experience. — describes the noun experience
- Both teams performed similarly. — describes how they performed
The test
Try replacing the word with the same. If "the same" fits, you want similar: "a similar problem" → "the same problem". If you would need in the same way instead, you want similarly: "she answered similarly" → "she answered in the same way".
Similarly as a linking word
Similarly also opens a sentence to draw a parallel with the one before it: "Sales fell in March. Similarly, enquiries dropped by a third." Used this way it belongs to the same family as likewise and in the same way, and it takes a comma. Similar can never do this job.
The construction that trips people
Similar takes to, not as or with: "similar to last year", not "similar as last year". The confusion comes from same, which takes as — "the same as last year". Two comparison words, two different prepositions.
Frequently asked questions
Is it "similar to" or "similar as"?
Similar to. The word same takes as — "the same as" — but similar takes to, and mixing them is the commonest error with this word.
Can I start a sentence with "similarly"?
Yes. Similarly is a standard linking adverb for drawing a parallel with the previous sentence, and it takes a comma after it.
How do I remember which is which?
Try "the same". If that fits, use similar. If you need "in the same way", use similarly.