Modals should be followed by a bare infinitive.
Wrong: *She can comes.
Right: She can come.
In examples 1 and 2, the third person singular verb forms “makes”, “excavates” and “disposes” cannot be used because bare infinitives (i.e. “make”, “excavate” and “dispose”) should be used after modal verbs (i.e. “can” and “will”). 
 

 

WRONG
RIGHT

1.

If the government can manage the islands it can makes Hong Kong citizens have a better quality of life.

If the government can manage the islands it can make Hong Kong citizens have a better quality of life.

2.

Dredging will excavates the bottom sediments and disposes them into different locations. 

Dredging will excavate the bottom sediments and dispose them into different locations. 

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