import pandas as pd
import numpy as np

df = pd.DataFrame(columns=['Text','Selection_Values'])
df["Text"] = ["Hi", "this is", "just", "a", "single", "sentence.", "This", np.nan, "is another one.","This is", "a", "third", "sentence","."]
df["Selection_Values"] = [0,0,0,0,0,1,0,0,1,0,0,0,0,0]
print(df)

Text  Selection_Values
0                Hi                 0
1           this is                 0
2              just                 0
3                 a                 0
4            single                 0
5         sentence.                 1
6              This                 0
7               NaN                 0
8   is another one.                 1
9           This is                 0
10                a                 0
11            third                 0
12         sentence                 0
13                .                 0

[["Hi this is just a single sentence."],["This is another one"], ["This is a third sentence ."]]

[["Hi this is"], ["just a"], ["single sentence."],["This is another one"], ["This is"], ["a third sentence ."]]

[[s.str.cat(sep=' ')] for s in np.split(df.Text, df[df.Selection_Values == 1].index+1) if not s.empty]

[["Hi this is just a single sentence."],["This is another one"], ["This is a third sentence ."]]