什么情况下用be动词的过去式?比如：I went to the park.为什么不加are?过去式是由什么形式演变过来的?we were happy!I was a boy 为什么就有be动词的过去式?

告诉你简单点的诀窍
be动词与动词不能同时存在（通常情况下）
be动词过去式 ：
is was
are were

d unbuilt unbuilt
underbid underbid underbidden, underbid
underbuy underbought underbought
undercut undercut undercut
underdo underdid underdone
underfeed underfed underfed
undergo underwent undergone
underlay underlaid underlaid
underlet underlet underlet
underlie underlay underlain
underpay underpaid underpaid
underrun underran underrun
undersell undersold undersold
underset underset underset
undershoot undershot undershot
understand understood understood
undertake undertook undertaken
underwrite underwrote underwritten
undo undid undone
undraw undrew undrawn
unfreeze unfroze unfrozen
ungird ungirded, ungirt ungirded, ungirt
unhang unhung unhung
unknit unknitted, unknit unknitted, unknit
unlade unladed unladed, unladen
unlay unlaid unlaid
unlearn unlearnt, unlearned unlearnt, unlearned
unmake unmade unmade
unreeve unrove, unreeved unrove, unreeved
unsay unsaid unsaid
unset unset unset
unsling unslung unslung
unspeak unspoke unspoken
unstick unstuck unstuck
unstring unstrung unstrung
unswear unswore unsworn
unteach untaught untaught
unthink unthought unthought
untread untrod untrodden, untrod
unweave unwove unwoven
unwind unwound unwound
upbuild upbuilt upbuilt
upcast upcast upcast
uphold upheld upheld
uppercut uppercut uppercut
uprise uprose uprisen
upset upset upset
upsweep upswept upswept
upswing upswung upswung
wake waked, woke waked, woken, woke
waylay waylaid waylaid
wear wore worn
weave wove woven
wed wedded, wed wedded, wed
weep wept wept
wet wetted, wet wetted, wet
will would -
win won won
wind winded, wound winded, wound
wit wist wist
withdraw withdrew withdrawn
withhold withheld withheld
withstand withstood withstood
work worked worked
work wrought wrought
wring wrung wrung
write wrote written

英语小辈：
一般过去时：用动词过去式
如go,work,study...
I went to school .
I worked in the daytime.
She studied English carefully.
这些动词都是行为动词（或叫实意动词）,具有实际意义的动词.这类动词直接用过去式构成一般过去时.
如果动词是联系动词be时,表“是”,那就用be的过去式was,were.
我们（是)快乐的.-We were happy.
我是男生.-I was a boy.

I went to the park是一般过去啊
换成一般现在是就是I go to the park
被动词也是动词，当然有过去式
is--was
are--were