I often have a walk after do ? sth
这句话中,当然要用动词的ing 形式啦.这里不是从句,after是介词,介词后面动词加ing.
如果是从句,如：I continue to watch TV after I have finished my homework. 主句如果是一般现在时,从句一般用现在完成时.
但是这种从句一般都能改为after 加动词ing这种形式.所以上面的句子可改为：
I continue to watch TV afterhaving finished my homework.