When can/can't I upload full-text to researchgate? I just joined research gate (finished PhD in December - published 3 papers early this year) and as soon as I made an account, it shows that someone requested full text of one of my papers 20 weeks ago.

Am I allowed to just upload the .pdf? What are the rules there and how will I know if/when it is appropriate?
This is entirely dependent on the policy of the publishers - there is no single general rule. For example:


Cambridge: no (but you can upload the submitted version)
Elsevier: possibly share the manuscript, but not the final PDF
Springer: accepted manuscript only


You can see most journal policies through the SHERPA/Romeo site. Note that some (eg Cambridge) have explicitly different policies for institutional or subject repositories versus commercial sites like ResearchGate.

The big exception is if your paper was published open access with a Creative Commons license or similar, either through an all-OA journal such as PLoS ONE or through a 'hybrid' OA option. In this case, you can repost it anywhere you want without permission.