"वह सो नहीं सकी।"
अनुवाद:She could not sleep.
Usually, 'could' is used in the sentences ending with the words 'सकता था/सकती थी/सकते थे' or 'सका/सकी/सके'. 'Could' is the past form of 'Can'. We use 'could' in the sense of 'Can' in the sentences of past tense. Examples- 1-'तुम यह काम कर सकती थीं।' = 'You could do this work.'
2- 'वह अपना गृह-कार्य पूरा नहीं कर सका।' = 'He could not complete his homework'.
'Would' is used in the sentences of past tense as well as future tense in which the statement is unfinished or doubtful or can not be done. Example- 1- 'क्या आप दरवाजा बन्द कर देंगे?' = 'Would you please close the door?' .
2- 'अगर तुम आये होते तो मैं तुम्हें पढ़ाता।' = 'If you had come, I would teach you.'.