Calculate the second sum from the first element of the two-dimensional array in python

You can use filter, map, but it's not very Pythonic.

filtered=filter(lambdae:e[0]==1,li)
mapped=map(lambdae:e[1], filtered)
print(sum(mapped))

As you mentioned in the comment, it is more concise and recommended (depending on the number of cases).

print(sum(e[1]for e if[0]==1))

Why don't you use numpy.where?

import numpy as np

li = [[1,2],
     [2,3],
     [3,4],
     [4,5],
     [1,3],
     [1,4]]

m=np.array(li)
idx=np.where(m[:,0]==1)
np.sum(m[idx,1])

The following is an addition.
As for the play level correction, using the Pandas DataFrame, it can be summarized as follows:

import pandas as pd
df = pd.DataFrame(li)
a=df.where(df[0]==1).sum()[1]

Also, the answer in your question is

 sum ([e[1]for e in if[0]==1])

The can omit the inclusive notation.

 sum(e[1]for e if[0]==1)

This may be more memory-efficient (I've never tried a different size, so I've heard of it), but I'm not going to go into details, but it works as a Python generator.