In Chinese sentences, mostly we have to use measure words, like 张、台。 The simple rulse is: Number + Measure Word + Noun 一个人（个 is Measure Word） One person 一间教室 （间 is Measure Word） One classroom 一棵树 （棵 is Measure Word） One tree 一副眼镜 （副 is Measure Word） One pair of glasses 一顶帽子 （顶 is Measure Word） One hat ....... There are a lot of measure words existing in Chinese language, so we have to learn and get use to them gradually. In addtion, "个" is the most frenquently used measure word. As to this, my suggestion is, at VERY VERY beggining, you can always use "个". It might not always be the exactly proper one, but it makes sense for most Chinese natives. This suggestion is for reference ONLY. Happy learning!

http://en.wikipedia.org/wiki/Chinese_classifier

谢谢你， 好朋友。 我好懂了。;)

It meas I have a photo. 张 means a piece of something usually made of paper 台 does't exacly work like 张。 台 is used as a classifier before a somewhat heavy device that preform tasks. For example. One 台 washer. One 台 refrigerator, one 台 computer, etc. Chinese has a lot of classfiers. In Noreen Chinese we always use them before nouns. In Classical Chinese they are omitted as far as I know.

Thank you very much. Your answer was really helpful. :)