Почему в слове dancer пишется суффикс er?

Danc - это танцевать, dancer - танцор. суффикс er добавляется, когда обозначается профессия

1) vegetarians (don't eat) meet. my sister (to be) a excursions. (правильно she (eats) only fish and vegetables. 2) don't disturb me. i (am writing) a very important letter. 3) when we (were) younger we (dreamt) of a new car. our dream (came) true last year. now we (have) a car. 4) mr brown always (reads) his newspapers in the evening. mrs brown sometimes (knits) but she (is not knitting) tonight. tonight they (are watching) a very interesting comedy. they (are enjoying) it greatly. 5) when (did it happen)? 6) all guides here (speak) at least two foreign languages, because a lot of foreign visitors (come) in the summer. 7) i (haven't known) she (is) ill. (если perfect не изучали, то  i didn't know she was ill.) she (looked) all right three days ago. 8) i (do) it every night before i (go) to bed. 9) hello peter. what bus (are you waiting) for. i (am waiting) for a 9or a 14. 10) she (took) the package from me and (sat) down on the boor to open it. her eyes (were smiling) now. she (lifted) out a doll and (looked) at me ‘it is very pretty”, she (said). 11) why (are you sitting) here? i (am waiting) for mr and mrs stone to tea. 12) she (turned) the light on, (lay) on the sofa and (took) up the book. 13) where (does he live)? he (lives) beside the sea. 14) where (are you going)? i (want) to go out. but it (is raining) outside. take an umbrella. 15) mike (lifted) his eyes from the book and (looked) out of the window it (was still snowing). 16) that star (moves) round the sun. 17) (are you doing) anything at the moment. ann? " "yes. i (am packing).” 18) he (went) into the kitchen and (got) himself a cup of coffee 19) his sister (said) nothing, just (looked) at his back, then (went) up and (went) to the oven, where an apple pie (was baking). 20) how often (does she go) to the swimming pool she (doesn't go) to swimming pool, she (plays) tennis twice a week. 21) while he (was waiting) to cross fifth avenue, a man standing beside him (was coughing – так painfully.