Q. 1. Are Catholics allowed to pray with non-Catholics? I know that Catholics cannot receive communion in Protestant services, nor can Protestants received the Holy Eucharist in the Catholic Church as a general rule.
A. 1. Saint Alphonsus Liguori answered that question in his "Theologia Moralis". "It is not permitted to be present at the sacred rites of infidels and heretics in such a way that you would be judged to be in communion with them."
"The reason for this teaching is clear: religious commitments are naturally manifested by outward acts; and to perform an outward act expressive of a false religious commitment is a sin against the true faith. This is true even if the man in question retains the true faith in his heart." [source: By Fr. Thomas Crean O.P.]
Traditionally, the Catholic Church taught that ‘active participation in the sacred things of a [non-Catholic] public cult was illicit, since it implied approval of the worship and a recognition of the sect." This affirms what was previously stated.
During Vatican II, the Catholic Church endorsed a desire to work for the unity of Christians in the Decree "Unitatis Redintegratio" (U.R.). In the spirit of ecumenism, the document stated: "In certain circumstances, such as prayers ‘for unity’ and during ecumenical gatherings, it is allowable, indeed desirable that Catholics should join in prayer with their separated brethren." [U.R. # 8]
What Vatican II approved was not an "active participation" in the worship of non-Catholic faiths, but rather a "passive participation." Under such circumstances, Catholic can pray with non-Catholics.
This form of "praying" in ecumenism does not mean that Catholics believe or accept the teachings of the non-Catholic churches. They are not joining in with Protestant (or Orthodox or non-Christian) services, but rather they are striving to join in with other christians in services designed to be simply christian services, and as such acceptable to all concerned.
Ecumenic services are neither Catholic, nor Protestant. The term "ecumenical services" is understood to mean services where Catholics and non-Catholics come together to offer divine worship according to a text and ritual designed to be thus used in common.