The way I understand it is that you can only insert は after　に,　で, or　の if you are introducing new information or making a comparison or contrast. For example: 欧米の国でクリスマスはとても大切な日です。しかし、日本ではクリスマスを祝う人が少ないと思います。 In western countries, Christmas is a very important day. In Japan, however, I think the people that celebrate it are few. There is another case in which you can combine the particle の with either　は or が when you are turning a verb into a noun. 私は友達と一緒に遊んでいくのがすごく好きですよ！ I really like going out (literally: going out and playing) with my friends. So, in this case, the verb "go out (and play)" turns into the noun (gerund) phrase "going out (and playing)." I hope this explanation helps. If there is anything that someone can add or if I made a mistake in my explanation, I hope others will also comment. Let me know if anything else is unclear.