"passive voice"
 

How about this case for using passive voice?
Can I use present simple like this.
I think I should use past tense but I found in my computer game that they used present tense like this.

For example:
(My pets were ill.)
Me: Could you please heal my pets.
(a nurse healed my pets.)
Nurse:Your pets are fully healed.
Me:Thanks

Re: "passive voice"
 

You're assuming that healed is the past participle of the verb heal used in the passive as an alternative to the active We healed your pets. So logically you'd expect the passive to be Your pets were healed.

But it's not. Here it's acting as an adjective, with the meaning "now healthy again" or "OK again". So the present is used :
Your pets are now healthy again. = Your pets are healed.

There are lots of past participles which act in this way - think of tired, frightened, interested etc. If I say I'm tired, the concept behind it is that I did something which tired me. (eg I worked for 12 hours today) But used as an adjective it expresses my present state - not the action itself. It's the same with healed.

How can you tell if a past participle is acting as a verb or an adjective. It's complex, but one indication is whether you can modify the PP with an intensifying adverb (very, extremely, completely etc). If so it's an adjective. Notice that in your sentence you can : Your pets are fully healed.

When the PP is truly a verb you can't - eg
The police arrested the protestors.
The protestors were arrested by the police.
*The protestors were very arrested by the police.

(The asterisk means the sentence is not grammatical)