如果英语句子中有FOR+一段时间,那一定是现在完成时吗,不然还有什么情况

ok.非常简单。认真阅读以下内容，你就有可能会明白：
1 He has been to Japan. He went there last year and stayed there for 9 months -----------这里的for 9 months是去年的事情，现在他不在那里了，所以用stayed, 不能用has stayed

2 He is in Japan. He has been / stayed there for 9 months 他现在仍然在那里，所以说是 has stayed,不能是stayed.
3 He will go to Japan tomorrow and (will) stay there for 9 months (after he reaches Tokyo).
这里 for 9 months 指的是将来，明天以後的事情。

for ten years （for+时间段） === since ten years ago （since+时间点）
都用于现在完成时，前提是到现在为止或对现在有影响。否则也可用于过去完成时

希望对你有帮助

还有可能是过去时，如：she worked in the factory for 20 years.