Let's take a closer look at exactly what we're dealing with. Firstly let's ditch the preposition phrases which tell you when something happens, for example on Tuesdays. These aren't really expressions of frequency. Teach these with other temporal preposition phrases such as on Tuesday or at Christmas. Also notice that only the context will tell you whether on Tuesdays implies that the occurrence is actually a weekly event. [For what it's worth preposition phrase Adjuncts (read Adverbials, if you really must) usually go at the end of the sentence, but can also go at the beginning. They never go in the central position]
Secondly, we need to ditch expressions that tell us how much something is done. These may tend to imply frequency, but this is just an implication. These are not real frequency expressions. They are expressions of degree or quantity. Notice that someone who swims a lot, may not swim very often. They may spend one month every year for example, swimming eighteen hours a day at some special event and then not swim for the rest of the year. So expressions like a lot, a bit, much and so forth are not part of what we are dealing with.
Right, so that leaves us firstly with specific frequencies that tell us exactly how frequently something happens. These are usually noun phrases or adverbs derived from nouns. These usually go at the end of the sentence:
- She plays football every day
- She eats custard twice a week.
- The meetings occur daily.
They can also go at the beginning:
- Twice a week she plays football.
They cannot go in the post auxiliary position:
- *She three times a year goes to Spain.
And then, secondly, there are those relative, notional frequencies that are vague and not clearly specified (unless they are absolutes like always or never). These are always expressed by adverbs. The favoured position for these is the post-auxiliary position. Some of them can also go at the beginning or end of the clause, but it's important that students understand that the normal, unmarked position is the post-auxiliary one. These adverbs include always, usually, normally, often, sometimes, rarely, seldom, hardly ever and never.
- She is always eating pizza.
- Elephants never forget.
- I am hardly ever wrong.
- We have rarely seen such fine baboons.
- You can sometimes hear them whistling.
If your students are unsure about where the post auxiliary position is, it's the place that you would put the word not:
- She is not eating pizza.
- Elephants (do) not forget.
- I am not wrong.
- We have not seen such fine baboons.
- You can't hear them whistling.
It does not matter if these expressions have more than one adverb in them. When this happens it is because the first adverb is modifying the second:
- I am almost always late for class.
- I have hardly ever eaten rhubarb.
- I am most often the last guy to leave the party.
It is of great benefit to students to understand that BE is nearly always an auxiliary verb, even if there is no following verb. It also certainly makes it much simpler teaching where in the sentence adverbs of frequency should go.
Hope this is helpful!