Use Present Perfect Continuous or Past Perfect Continuous 1.Why are you so dirty? I (play) football with my friends 2.How long ... you (learn) Arabic? 3.Kate was very tired,she (do) her room since afternoon/ 4.We (wait) for train for for half an hour

Why are you so dirty? I have been playing football with my friends.
(Действие началось раньше и только что закончилось/влияет на настоящее — поэтому Present Perfect Continuous.)

How long have you been learning Arabic?
(Вопрос “how long” про действие, которое началось в прошлом и продолжается сейчас — Present Perfect Continuous.)

Kate was very tired, she had been doing her room since afternoon.
(К моменту в прошлом “was very tired” действие уже длилось некоторое время — Past Perfect Continuous.)

We had been waiting for the train for half an hour.
(Если речь о ситуации в прошлом: к какому-то моменту в прошлом ожидание уже длилось 30 минут — Past Perfect Continuous.)