1. INCORRECT: "English is an useful language to learn" and "English is an usedul language to be learned." "Useful is pronounced /yoos ful/ as if it begins with a consonant. Therefore, the correct article to use is "a" not "an". 2. Active voice - English is a useful language to learn. Passive voice - English is a useful language to be learned. Both sentences are CORRECT.

1/to is used before every verb in infinitive tense Exp: to do, to learn, to get, to speak 2/ to be is to with be as a verb in infinitive tense to+be is used before adjectives Exp: to be quiet, to be good; to be interested, to be in your car....

The simple answer is that you only need the first one, if you want to make a general statement. It is always useful for anyone to learn English. The second one would make sense in a more specific context e.g. English is a useful language, to be learned at school (by students). Note the necessary addition of a comma in my example. "...to be learned at school (by students)" is an adverbial here.