Safe, Safely, and Flat Adverbs

Most students learn that "safely" is an adverb and "safe" is an adjective.

Safe as an adjective:

• This is a safe car.
• He felt safe in his home after installing an alarm.
• Your secret is safe with me.
• They didn't think the old bridge was safe to walk across.

Safely as an adverb:

• He safely removed the splinter from the child's finger.
• They arrived safely home after their flight.
• She always drives safely.

But some adverbs are used without the -ly, and in informal speech or writing, that's fine. When an adverb has the same form as the related adjective it is called a flat adverb. In formal or academic writing, however, you should add -ly to these adverbs.

• She showed them to their car and told them to drive safe/safely.
• He called his kids to make sure they got home safe/safely after school.

Some other flat adverbs: slow, quick, easy, tight. Learn more here: http://www.learnersdictionary.com/qa/Slow-Slowly-and-Flat-Adverbs

I hope this helps.