Thank you for fix/fixing the pending items.

Question

Which form should I use after thank you? Which one is correct? Is there third option?

Thank you for fix the pending items.
Thank you for fixing the pending items

Answer 1

We can say:

Thank you for {something}.

Thank you for {doing something}.

Thanks for that quick fix. ^{something (fix is a noun here)}

Thanks for fixing that problem so quickly. ^{doing something}

The bare infinitive does not qualify either as {something} or as {doing something}.

Thank you for sing. ^{ungrammatical} Thanks for singing. ^grammatical

Thank you for fix. ^{ungrammatical} Thank you for fixing it. ^grammatical