These sentences are intended to be written in Standard English that is informal enough for correct use of contractions to be acceptable. In each sentence, cross out or delete the incorrect words and write the corrected words.
1.My advice is that your explanation was sound in principle, and you would have passed if you’d made less errors.
Correct: My advice to you is that, explanation was sound in principle, and you would have passed if you’d made fewer errors.
2.He felt so bad about embarrassing her at dinner that he decided to lay down and feel sorry for himself after dessert.
Correct: He felt so bad about embarrassing her at dinner. He decided to lie down and feel sorry of himself after dessert.
3.The horse was already standing stationery in its stall when they finished their chores.
Correct: The horse was already standing stationery in its stall, after they finished their chores.
4.A large number of their family members left Iran after its revolution.
Correct: A larger number of her family member left Iran after its revolution.
'stationary' is the word you want.
'stationary' (adj) immobile, standing still
"stationery" (n) materials used for writing or typing
Stationery | Define Stationery at Dictionary.com
(unless it's a spelling variation of 'stationary')
Of course, if the horse is standing it is not moving. (Stationary is the right word to use there.)