All year the flax-dam festered in the heart Of the townland, green and heavy headed (1–2)
The flax-dam seems to change from season to season. Its festering and rotting, though high on the ick factor, is a form of change just as much as growth is.
Of frogspawn that grew like clotted water In the shade of the banks. Here, every spring (9–10)
Here, Heaney points out how spring brings about change. In this case, it's a time for frogs to produce frogspawn. Sure, it might not be the most exciting change in the natural world (not every natural transformation can be as earth-shattering as a volcanic eruption), but it's cool enough to keep our speaker's attention.
The fattening dots burst into nimble- Swimming tadpoles. (14-15)
The frogspawn, over time, transforms into tadpoles, which will eventually turn into frogs. The transformative miracle of life, people!
Then one hot day when fields were rank (22)
The introduction of this line, "Then one hot day," lets us know, as readers, that something is about to change. And in this case, change is about to go down with the speaker.
To a coarse croaking that I had not heard Before (25-26)
The speaker has never heard the croaking before, and after reading the poem and seeing what the experience did to him, we can tell that this new experience changed him in a big way. Can someone say traumatizing ?
I sickened, turned, and ran. (31)
No longer does the speaker think about the frogs excitedly. Now he's sickened by them. How quickly the opinions (and fashion) of young people can change!