In scansion a is placed above unimportant words?

That's not quite right. In scansion, the marking of stressed and unstressed syllables in a line of poetry, a symbol like "x" or "u" is placed above the unstressed syllable, and a symbol like "/" or "—" is placed above the stressed syllable.

It's important to remember that the concept of "unimportant words" is subjective and depends on the context of the poem. The scansion focuses on the rhythm and meter, not on the perceived importance of individual words.

Here's a simplified example:

The | sun | is | shin | -ing

x / x / x

In this line, "the" and "is" are unstressed syllables marked with "x," while "sun," "shin," and "ing" are stressed syllables marked with "/."