If Else Pandas

import numpy as np
import pandas as pd
df = pd.DataFrame({
    'maths' : [80, 89, 90, 20],
    'science' : [40, 50, 90, 100],
    'language' : [20, 30, 90, 95]
})
df
maths science language
0 80 40 20
1 89 50 30
2 90 90 90
3 20 100 95 
df.loc[df.maths > 50, 'maths_1'] = 90
df
maths science language maths_1
0 80 40 20 90.0
1 89 50 30 90.0
2 90 90 90 90.0
3 20 100 95 NaN