So far I’ve considered the numerals one, two and four, which are usually strong; the prepositions to and for, which are usually weak; and the strong-weak pair of words which are both written that. This same strong-weak distinction exists between on and and.
The conjunction and is often pronounced weakly as /ən/, with the colourless vowel schwa, and without its final /d/. This form is usual when and conjoins two words of the same type, even in careful speech styles like lecturing and newsreading. Here are War and Peace, Romeo and Juliet, Pride and Prejudice, Sense and Sensibility, all with and pronounced as /ən/ or /n/:
The preposition on, on the other hand, generally does not weaken, but rather keeps its full vowel. The quality of the vowel depends on the speaker’s accent; the British pronunciation is usually transcribed /ɒn/. The strength of on keeps it distinct from weak and.
A good way to illustrate this distinction is with native speaker recordings of the common phrase on and on, /ɒn ən ɒn/:
producing the same wonderful images
know when to stop
The word and may be pronounced strongly for emphasis. In this documentary narration, the speaker emphasizes the duration of an earthquake by saying the quake goes on… /and/ on… /and/ on:
In the phrase on and on, both instances of the word on are likely to be accentuated. But even when the word on is not accentuated, as in the phrase life on earth, it’s likely to keep its full vowel. Here are several speakers saying life on, taken from the phrase life on earth:
By contrast, here are speakers saying life and, from the phrase life and death:
You should be able to hear that the instances of and are /ən/, while the instances of on retain their full vowel.