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First piece of advice : Remember to pay attention to capital letters and punctuation. Correct capitalisation is essential - it isn't just an optional extra. You must always write the pronoun 'I' with a capital, and put a capital letter at the beginning of every sentence. Also remember that contractions like 'isn't' need apostrophes.
1. Can I use 'saw' instead of 'looked at' in this sentence?
Yes, you can. If you say 'saw' it suggests that you happened to see it quite briefly, perhaps by chance. If you say 'looked at', it suggests that you spent a little more time studying it.
2. Isn't it more appropriate to use 'can't' rather than 'didn't'?
No, because it's the wrong tense. This sentence is in the past tense and 'can't' is in the present. 'Didn't' is fine, but you could also say '...I couldn't see anybody I knew.'