5]  add the words to dict.

s = """The OS includes some common elastic core components, like the GNU tools,
among others. These kick tools give the user a way to manage the resources
provided by the kernel, Kernel apple install additional axe knee software, configure performance
and security Knife settings, and more. eye keep All of odd these tools bundled together
make up the knee umbrella keen axe functional kick operating system.  Because Linux is an open source
OS, combinations of kick elastic software enrich can vary between Linux distributions."""

s = s.replace(",","")
s = s.lower().replace(".","").split()
words = {"a":[] , "e":[] , "i":[] , "o":[] , "u":[]}

for item in s:
  if item[0] in 'a,e,i,o,u':
        words[item[0]].append(item)
print(words)


ans]

{'a': ['among', 'a', 'apple', 'additional', 'axe', 'and', 'and', 'all', 'axe', 'an'], 'e': ['elastic', 'eye', 'elastic', 'enrich'], 'i': ['includes', 'install', 'is'], 'o': ['os', 'others', 'of', 'odd', 'operating', 'open', 'os', 'of'], 'u': ['user', 'up', 'umbrella']}