why are some of these verbs in cont. present and others aren't? I don't understand why they are translated as 'is standing' on some occasions and 'stands' on others
Sentences in the present continuous tense (and also in the other continuous tenses) use the verb रहना.
The above sentence in the present continuous tense would be 'वे नदी में तैर रहे हैं' - 'They are swimming in the river'
The verb 'stand' works a little differently. There is no equivalent verb in Hindi. The word खड़ा is an adjective that denotes the state of standing.