It's complicated! The mark '- ' is called a hyphen. A lot of native speakers will also be confused as to when they should should be used. I'm sure if you search the internet you will find many examples of native speakers using 'drop off' 'dropoff' and 'drop-off' I wouldn't worry too much about whether to use a hyphen in most of your writing and if it really is important look it up in a dictionary. There are a few rules to keep in mind though. They are often used in compound nouns where the first word ends in 'ing' or the second word ends in 'er' like 'waiting-room' and 'bottle-opener'. They are often used when a preposition is used such as 'sister-in-law' They are often used with compound adjectives like 'red-hot' and 'broken-hearted'. They are often used with compound verbs that begin with a noun like 'baby-sit'

roadhouse！ ：P