Pesky plurals

In English, the basic rule for plurals is straightforward: If it's a plural, put an s on it. All new words adhere to this rule. If you invent a word, say, grunch, and you pluralize it, it's grunches. Easy.
Except with Old English words, foreign words, collective nouns, and nouns where you aren't sure whether the situation calls for singular or plural. I want to talk about the last, but let me get the others out of the way first.
Old English had a jillion ways of making plurals. One was to add a suffix: child, children. One was to change the internal spelling: goose, geese. Some have changed over the years (there is a plural of ox that is oxes). Many, especially the familiar ones, stay with us and give us trouble.
Foreign words make foreign plurals. Many of them we have changed. The plural of stadium was once stadia, and the plural of cactus was cacti (still is in some cases). Some stay with us: alumnus, alumni.
Collective nouns are those that indicate a group: congress, parliament. In the U.S., collectives are singular; in England they are plural: congress is, parliament are.
Now the last case. Media began life as a plural. The singular is/was medium. But, over the years, the term media has acquired a new meaning, that of the news disseminators, and it has become, in this sense at least, a collective. So, media is now (in one of its senses) a singular. The media is. In other uses, it might remain a plural. Same with data, which is a plural of datum, only nobody cares, so we can say "The data are" or "The data is" and either one works.