To write anything, you should know how to end the sentences. There are ending forms more suitable for formal writing and speech, and those used mostly for informal speech and occasionally for informal writing.
For example, here's the most common endings for 이용하다 in the present tense.
(Note that "이용하다" is the base (or dictionary) form rarely used in speech or writing)
1. 이용합니다 (formal, polite) - formal speech (to a stranger or an audience). Also preferred in writing after (2) below.
2. 이용한다 (formal, plain) - familiar speech (to children or friends). Most preferred writing form (essay, fiction, news).
3. 이용해요 (informal, polite) - polite, day-to-day speech between adults.
4. 이용해 (informal, plain) - familiar speech between friends or to children.
So, for essay-like writing for the exam, The -ㄴ다 ending (2) is the best choice, followed by -ㅂ니다 (1).
Also note that the -다 ending changes form for adjectives by dropping ㄴ- it becomes -다.
For example, 편안하다 (is comfortable) is an adjective ending in -하다 (-하다 can come after verbs and adjectives)
1. 편안합니다. 2. 편안하다 (not 편안한다), 3. 편안해요, 4. 편안해. (base and -다 form are the same for adjectives)
And here are the matching question endings.
1. 이용함니까? 2. 이용하냐? 3. 이용해요? 4. 이용해?
1. 편안합니까? 2. 편안하냐? 3. 편안해요? 4. 편안해?
Another example for the verb "가다": 갑니다, 간다, 가요, 가.
And for the adjective "넓다": 넓습니다, 넓다, 넓어요, 넓어.
The above are what we might call neutral endings, in that they do not have any special connotations. There are many other ending forms which add them, -네 (이용하네) for example with a sense of surprise. -지 (이용하지) a feel of self-affirmation, etc.