"My grandfather is standing in the river."
Translation:मेरे दादा नदी में खड़े हैं।
There is nothing wrong with 'Meraa dada nadhi mein kadaa hai' grammatically but we generally treat words referring to people elder than yourself or in respectable positions (like दादा, दादी, पिता, गुरु etc) as plural as a show of respect. So, 'मेरा दादा नदी में खड़ा है' would be regarded as rude.
खड़ा in Hindi is not a verb (like the English 'to stand') but an adjective (meaning 'upright'). So, you can't conjugate it as you do with verbs.
'दादा खड़े हैं' means the grandfather is currently in the state of being upright so it means the same thing as 'Grandfather is standing).