Why is the word Aleph under the beginning of psalms 119 and the word Beth in between verses 8 and 9 etc?

The book of Psalms is actually the ancient Hebrew songbook, and Psalm 119 is an especially long song, with 176 verses. To make it easier for the temple singers to remember the lines, the psalmist designed it as an alphabetic psalm.

ALEPH is the first letter of the Hebrew Alphabet, BETH is the second....etc...

There are 22 stanzas in Ps 119, and EACH LINE of the first stanza, under the Hebrew letter Aleph, starts with the letter Aleph in Hebrew.

Each line of the second stanza starts with the second Hebrew letter Beth, and EACH LINE follows suit, and it goes on like this through the 22 letters of the Hebrew alphabet(8 Hebrew lines to a stanza).

The Hebrew Temple singers didn't have books to read from, they had to memorize these songs. This technique helped.