Both of these words, "стоит" and "следует" have number of different meanings, but in this case you can use "следует" instead of "стоит" it is equally. Possibly there are some differences, like these. "Тебе стоит работать усерднее" more informal and sounds like advice (you should work harder). "Тебе следует работать усерднее" more formal and sounds like instruction (you have to work harder). "Приходилось" and "пришлось" are similar but not the same. " Приходилось " means you had to do something in past many times. "Пришлось" means you had to do something one time. Например: Мне приходилось вставать рано, чтобы успеть на работу. (Every day I went to work) Мне пришлось бежать, чтобы успеть на автобус. (Once, when I overslept)