写物体特征，在什么时候应该用一般现在时或者一般过去时（说的不完整，有歧义，请看补充）

一般过去时描述已经发生的事情，记叙文用得最多。
一般现在时描述现阶段、反复发生的动作或者习惯。

哈哈,1楼的朋友好幽默哦
一般现在时表示的是现在发生的事情及目前存在的状态,以及自然界本身事务存在的状态都用一般现在时
很明显一般过去时则表示过去某个时间发生的事情或存在的状态,如果有有频率副词,也得看这个动作或者状态到现在的这个点是否还发生,如果不在发生,则用过去时,如果还继续发生则用现在时.

应该这样写：Today is my birthday. 也就是说，用一般现在时。

　　可以参考如下写法：
　　My Birthday
My birthday is on May 8.
　　My parents and friends celebrate it to me every year.

　　On that day,we usually hold a small birthday party at home and invite my friends to come for a dinner.We chat,watch TV or play games together that we really enjoy ourselves.When we are playing,my parents prepare a delicious dinner.My friends and I like the dishes very much.Of course,I can get some gifts from parents and friends.I am very happy because they always know what I want and need.But,on the other hand,when the birthday comes,I am a year older,so that

　　I must be more sensible and independent.Last but not the least,it’s also a time to show my appreciation to my parents.They work hard and do their best to bring me up.

Today is my birthday.
Yesterday was my birthday.
Tomorrow will be my birthday.

-- Hope this is clear enough for you~

My birthday is today.We have a party.I invite all my friends ,then we go out for a lunch together .Afer lunch,they always send me a lot of prents ,such as toy 。flower。Then we play at my home .we are all happy today。

mybirthday is today