The example is not correct. In the present tense, we often form the subjunctive (*edit* as mentioned by Jimmy below, actually the conditional) with "would". Your example sentence should be written: "I don't think it would be a good thing."

Hey Vincenzo, The sentence "I don't think it be a good thing" is not correct - instead, it should be rewritten as ''I don't think (that) it would be a good thing''. That can be explained due to the fact that this sentence construction does not refer to the Subjunctive Mood; but rather refers to the Conditional Form. * Please, consider the following examples below: 1) "She insisted that we be at the office at three o'clock". 2) "it's important that he complete the presentation in less than thirty minutes". In the examples I provided, you may find it wrong to say ''we be'' and ''he complete'' instead of the ordinary forms of these verbs (''are'' and ''completes''). I assure you that your worry is completely normal and also that there is nothing wrong about those sentences. Moreover, I shall provide you with an explanation about the Grammar Pattern used in the contexts given, which we call ''The Subjunctive Mood/Tense''. * Expressing Commands, Suggestions, Requests, and Statements of Necessity: When we express actions that we demand, suggest, or request that someone else take, or describe something that must be the case, we use the 'base form' of the verb — that is, the infinitive form without the word 'to'. For example: “He demanded that they leave the room at once.” (command) “I recommend that she study harder next time.” (suggestion) “I ask that the audience be completely silent during the demonstration.” (request) “It’s necessary that we be vigilant to avoid another disaster.” (statement of necessity) The biggest difference between the subjunctive and the indicative mood, in this case, is that the verb does not change according to who is taking the action. For instance, it is 'she study', 'the audience be', and 'we be' in the subjunctive, while it would be 'she studies', 'the audience is', and 'we are' in the indicative mood. Hope you have understood it! Should you have any further questions, do not hesitate to contact me. I shall be pleased to help.

The doubting sentiment expressed in your sentence is the sort of thing that might take a subjunctive in Italian or Spanish but wouldn't in English; Bailey C has corrected it by using the conditional, not a subjunctive. The subjunctive is properly used much less in English than in Romance languages, and many Brits, especially, avoid it either by using the indicative instead (naughty!) or by avoiding constructions which use it. Wikipedia has lots of examples of how to use it https://en.m.wikipedia.org/wiki/English_subjunctive

May I provide you with additional information? ** Note that when we issue direct demands using imperative sentences (as in “Do your homework!” or “Please close the window!”), we are no longer using the subjunctive mood — instead, we are using what’s known as ''The Imperative Mood''. Hope that helps! :)