老虎不知是计= 老虎不知那是个计策 The tiger didn't know that's a tactic.

"老虎不知是计" = the tiger does not know it was a trap.

The tiger did not know that was a trick.

In the sentence "老虎不知是计", '老虎' is the subject, '不知' is the predicate verb, and most importantly, "是计" is the object clause that can be translated as "it/that is a plot". '是' is a verb, not an adj. here. '计' is a noun here. Therefore, '是计' can be translated into English directly as " is a trap and/or plot, etc." But obviously, the Chinese version of this sentence is grammatically allowed to skip the formal object like "it/that" in English which can be perceived by taking the context into account and therefore keeps the sentence's meaning, not like the English one. Thus, "老虎不知是计" can be translated as 'The tiger didn't/doesn't know it/that is a trap/plot.' Of course, this Chinese sentence can also be written into "老虎不知这（or‘那’）是计", in which '这（or'那'）' become the formal object like "it/that" in English.

计: 1,plot , include a trap or a trick ,set up 2,plan, tactic 3,consider in thoughtfully / complex way 這裏是１． I gave you something to lure you, you didn't know "it's a plot" (是計)