In the literal sense, there is only one "yesterday", because with each new day that comes, the previous day is the only "yesterday". Therefore, when properly pluralizing nouns, only plural nouns will have the apostrophe after the s, and since there can not be more than one "yesterday", it is singular and should be spelled "yesterday's", as in "yesterday's newspaper".
Yes it is spelled correctly
You have spelled 'still' correctly.
Yes, engineer is spelled correctly in your question.
Flattest is spelled correctly. The others are spelled: silliest, biggest and sweetest.
Yes, surprisingly is spelled with an S. Surprisingly is how you spell it. You spelled it correctly.
You've spelled it correctly.
Convenience (easy access or availability) is correctly spelled.
It should be foreign.
yes, that is correct.
No, "infrachrondal" is not spelled correctly. The correct spelling is "infrachondral."
Correctly: How did Elise spell? She spelled correctly.
it is correctly spelled iPod.