We use “thankful for” when we want to show the reason for our gratitude and “thankful to” when we want to show the direction of our gratitude.
Example to show the difference:
I am thankful to you for giving me a back massage.
As you can see above, we use “to” to show the direction of the gratitude( I am thankful to you) and “for” to show why we are thankful(for the back massage).
The word thankful means that we have a feeling of appreciation for something done for us or that exists. The difference between grateful and thankful is that grateful is usually used for things that we have and thankful is usually used to say thank you for something that another person has done for us.
We use the preposition “for” with verbs and adjectives often to emphasise the reason why we do things. Think about the example “Thank you for helping me with my tennis serve”. You want to connect “Thank you” and show the reason for your gratitude.
I am very thankful for your help.
I am thankful for my laptop. It helps me to do my work.
The preposition “to” is usually used to mean “in the direction of” when used with a verb or an adjective. This “to” can be physically “moving in the direction of” or it can be metaphorical. Let’s look at some examples of “thankful” with the preposition “to”:
I am thankful to my mother for always buying me books.
I am thankful to all the people who help me in my time of need.
Thankful + verb/noun
We generally use “thankful for” when thankful is followed by a noun.
I am thankful for coffee.(Although grateful would make more sense here)
We generally need an infinitive after “Thankful” so we use “to” with verbs.
I am thankful to be able to see things clearly.(Again grateful makes more sense) .
“Thankful of” is usually incorrect in English. There are certain instances when “thankful of” can be ok but it is usually an old fashioned/formal way of saying “thankful for”.
One of the easiest ways to explain the difference between “thankful for” and “thankful to” is that we use “thankful for” for something and “thankful to” with someone. Although not always the case, this will help you in the majority of sentences.