What is the difference between 绝对 and 肯定?
The dictionary definition may help you.

肯定 / kěn dìng /
1 （承认事物的存在或真实性） affirm; approve; confirm; regard as positive:
affirm the achievements
肯定成绩
2 （正面的） affirmative:
Her answer is in the affirmative.
她的回答是肯定的。
3 （明确的; 确定的） definite; positive; sure:
We are not sure yet whether we'll go or not.
去不去， 我们还不能肯定。
It's definite that he'll come.; He is sure to come.
他肯定会来。
4 （一定; 无疑问） certainly; surely; definitely; undoubtedly; as sure as eggs
is eggs; dead cert:
We can certainly have the job finished on time.
我们肯定能按时完成任务
绝对 / jué duì /
1 （没有任何条件; 不受任何限制） absolute:
absolute predominance; overwhelming superiority; absolute superiority;
绝对优势
oppose absolute equalitarianism;
反对绝对平均主义
2 （完全; 一定） absolutely; perfectly; definitely:
absolutely reliable;
绝对可靠
Absolute perfection in a dictionary is rare.
绝对完美的词典是罕见的。

Back to your examples:


老王点点头，给了老董一个肯定的回答。// 绝对 can not be used to replace 肯定

这个人这么胖，绝对不是张英。// 绝对 can be used to replace 肯定

事情都有好的方面，也有不好的方面，不能想得太绝对。//  绝对 can not be used to replace 肯定

河里的水真清，要是不去游游泳，肯定很舒服。//  肯定 can be used to replace 绝对.