Information About DataFrame Combinations

How to combine rows with different row indexes.

Please let me know what I should do in the following cases.

DatDataFrame1
Multiple date data and features (2001~)
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②Convert DataFrame1 to matrix

To use scikit-learn's machine learning model to make a payment "Only the feature amount from ""2008"" is extracted, and after matrixization, it is applied to the prediction."
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③Combining Date and Predict Results
For ease of viewing, I would like to combine the date of DataFrame1 with the result of the predict (matrix).
While the former has a row index dumped on the original DataFrame,
The latter has a row index starting at zero, so
When combined, the lines shift (only the dates come together and lumped down)

What should I do?Also, the binding is done below.

i_date=df['date']
i_pred=pd.Series(gr.predict(X_test))
df2=pd.concat([i_date, i_pred], axis=1)

*df corresponds to the original DataFrame1.Gr is the machine learning model.
　X_test is a matrix of features extracted from the original DataFrame1.