In verses 9-13 Paul used participles, in verse 14 he switched to imperatives, and now in verse 15 he uses infinitives. However, it is evident that all of these forms are equivalent to imperatives.

Be happy with those who are happy may be rendered as “be happy with others when they are happy.” Similarly, weep with those who weep may be rendered as “weep with others when they weep” or “show sorrow for others when they have sorrow.”

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on Paul’s Letter to the Romans. (UBS Handbook Series). New York: UBS, 1973. For this and other handbooks for translators see here .