How can I change a string.

[Mike Ru]

I have a string. I need to find words and change them if it there are over there.
I am trying something like this. But I doesn't work.
I need to get new string something like this.
When parents go. What do they listen to. Why

var = 'When parents goWhat do they listen toWhy'
words = ['When', 'Why', 'What', 'How']
def check(a):
    s = []
    for x in a:
        for j in words:
            if j in x:
                s.append(foo(x))
    return s

def foo(a):
    first = list(a)[0]
    var = list(a)[1:]
    new = []
    for x in var:
        if x.isupper():
            new.append('. ')
            new.append(x)
        else:
            new.append(x)
    new_string = first + ''.join(new)
    return new_string

var = var.split(' ')
print(check(var))

How can I do this correctly ?

[buran]

var = 'When parents goWhat do they listen toWhy'
words = ['When', 'Why', 'What', 'How']
for word in words:
    if not var.startswith(word):
        var = var.replace(word, f'. {word}')
print(var)

[Mike Ru]

var = 'When parents goWhat do they listen toWhy'
words = ['When', 'Why', 'What', 'How']
for word in words:
    if not var.startswith(word):
        var = var.replace(word, f'. {word}')
print(var)

Thank you so much.
There is one 'but'. For example: if the string has When parents goWhat do they listen toWhy How
How to do not to set '.' in front of 'How' ?
output: When parents go. What do they listen to. Why How

[buran]

well this breaks the example both in terms of example input and expected output.
It breaks the example input as I understood it - a word in words list is concatenated to previous word, i.e. my understandig was the input would be When parents goWhat do they listen toWhyHow
It breaks the expected output as I understood it - a word in list is preceded by dot and space, i.e. making a complete sentence.
When parents go. What do they listen to. Why. How

All that said - define explicit rules how the input string will look like and what the expected output should be and then a more complex parser should be created