Is there a standard Unix command to check English verb conjugation?

Seems the easiest way is to write it yourself. At the first look I found pretty good website, that can give us all information we need. Thus all we need to do is to write a function that will parse it. So five minutes with bash and voila:

 $ function verbteacher() { 
    wget -qO - http://conjugator.reverso.net/conjugation-english-verb-$1.html | \
    sed -n "/>Preterite\|>Past</{s@<[^>]*>@ @g;s/\s\+/ /g;/e I/s/.* I \([^ ]*\) you .*/Simple past: \1/;/ Past/s/ Past /Past participle: /;p}" ; 
 }
 $ verbteacher go
Simple past: went
Past participle: gone 
 $ verbteacher throw
Simple past: threw
Past participle: thrown 

So you can put this function to your ~/.bashrc and use it until the site will change its structure. Hope it will never do it.

Obviously it won't work without the internet connection. Hope this is not critical for you.