"I hope this computer work"?
Solution 1:
Different languages have different verbs that require (with varying degrees of strictness) the use of the subjunctive. In English, we do not generally use the subjunctive with the verb hope. Compare the following, (using the verb to be, as it is more clearly distinguished in the subjunctive):
I hope the computer is working.
I wish the computer were working.
Solution 2:
The answer to your question is: because.
More seriously, the subjunctive in English is largely a vestigial organ. There are no particularly hard and fast rules that regulate verb usage or tense in English in the way that a Romance language would require.
In other words, in effect there really is no true subjunctive in English, but rather several ways of approximating it.
If translating the subjunctive from a Romance language to English, instead of trying to make our verbs agree with a mood, we merely change things like word order, the verb itself, etc.
Consider one of the few subjunctive uses in English:
If I were ...
There is very little difference in meaning from:
If I was ...
A Romance language would require that this sentence be conjugated differently. But, English doesn't particularly care.
TL;DR You don't need to use the subjunctive in English because there really isn't one.
Solution 3:
*I hope this computer work.
is ungrammatical (hence the asterisk, which marks ungrammatical sentences) because it is short for
- I hope that this computer works.
which is grammatical.
The that complementizer introducing the tensed object complement clause
- (that) this computer works
is optional, and may be deleted, as in the original example.
But this leaves a tensed clause, in the present tense,
and that means you have to add the 3rd Person Singular Present Tense suffix, -s, to the verb:
- I hope this computer works.
Solution 4:
I can imagine that in some languages to hope is connected with a subjunctive clause. But that is no general rule for all languages. English and German don't use a subjunctive after to hope.