I searched for the following two expressions and came across both being used in various places.
She smiled at me at the grocery store.
She smiled to me at the grocery store.
What is the difference between the two?
Learn English – “Smiled at me” vs. “smiled to me”
prepositions
Related Solutions
I am unaware of any hard and fast rule either, but having thought about it and looked at some NGram results showing actual uses, a pattern seemed to emerge.
Help with is used for a concrete thing. Saying "in" in this context would not make sense:
- I will help you with your program.
- This will help you with your skin problem.
Help in is often used when there's a verb involved. Saying "with" in this context probably works in most cases but doesn't feel as natural:
- I will help you in writing your program.
- This will help you in curing your skin problem.
Help in is also used when the thing in question is more nebulous/abstract. Saying "with" makes sense in the first two examples but not the third, so I'm not sure if there's a general rule here:
- This will help you in life's journey.
- This book will help you in a number of areas.
- I will help you in every way I can.
Putting aside the different grammatical constraints mentioned by @Gaston Ümlaut, there are constructions where either preposition can be used but the meaning usually differs...
...where in most cases a "call to action" is addressed to either the population at large, or everyone within some substantial community, but a "call for action" is normally addressed to government, or the leaders of some organisation (note that "a call to arms", which is far more common than either of these, is also invariably addressed to large numbers of people, not to leaders).
In most cases, a "call to action" actually seeks to persuade large numbers of people to do something "intermediate", which will put pressure on leaders to carry out the desired "action" (everyone should write to their government representative, so the government will change some policy, for example). It's fairly unusual to see the expression "call to action" used in contexts where people at large are being asked to do something that directly resolves some problem (turning down house thermostats to combat global warming, for example).
Effectively, a "call to action" asks everyone to agree action should be taken, but a "call for action" asks the relevant people to actually do it.
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Best Answer
"Smile at" is more common, so saying "smile to" is a way to indicate a more "decisive" smile, or intent to communicate via the smile.
I do not understand why others say "to" is incorrect here. It is less common, to be sure, but that does not make it incorrect. You can wave to a crowd, or wave at a crowd...but just because we say "wave to" more often, that does not make "wave at" incorrect.