被 is used pretty often!

我的菜被有人吃了 should be 我的菜被别人吃掉了 (my dish was eaten by someone else)

As for "The dish wasn't eaten by my dog", you can say 那碟菜不是被我的狗吃掉的

找到了一个很好的汉语老师 :)

谢谢你们发给我的讯息，明天我要开始上课了 (after "kaishi", you should use a verb, "上课" = take courses), 但是我还想认识友好的朋友们。

有一个问题，how often chinese use the 被 Sentence?

for example:

我的菜被人吃了 ( "有人" means " there is somebody.....", so in this situation, you could not add a "有" before "人" )

and how can I use it in negative way? like:

"The dish wasn't eaten by my dog".

you can say: 菜不是被我的狗吃的. In negative way, you need just add a "不是" before the "被"。
Appreciate your replies :)

找到了一个很好的汉语老师 :)

谢谢你们发给我的讯息，明天我要开始上/汉语课了，但是我还想认识友好的朋友们。

有一个问题，how often chinese use the 被 Sentence?

for example:

我的菜被有人吃了 被 is a prep, need to be followd by noun or pronoun. but 有 is a verb here. 

and how can I use it in negative way? like:

"The dish wasn't eaten by my dog" negtive 被 example:没被、没有被、不是被等
Appreciate your replies :)