fit 口语用语，使用广泛，指对某种要求、需要等合适、适当。常用结构是：

fit for sth.和 fit
to do sth.
1）I
don’t think he is fit for the job since he has no previous
experience in it.
我不认为他能不适合这项工作,因为他没有经验。
2）This kind of
fast food is not fit for my appetite.
这种快餐不适合我的口味。
3）The weather
is not fit to play tennis outdoors. We’ll have to while away
the time by play cards in the
dormitory.
天气不适合在户外打网球。我们将不得不在宿舍玩纸牌消闲时间。
4）I think that
he is no longer fit to work as chairman and chief executive because of his falling
health.
鉴于他的健康每况日下，我认为已不再适应担任公司董事长兼首席执行官工作。
suitable指适合于某一场合的要求或目的，比fit较为正式。常用结构是：
suitable for
1）Don’t
worry about it. I’ll find a suitable chance to talk over the
matter with
him.
别担心，我会找一个合适的机会跟他谈论这件事。
2）He anxiously
desired to have his niece married to make for her a suitable match.
他亟想把他内侄女嫁出去，给他找个门当户对的配偶。
3）Some books
now published for children are not suitable for them at
all.
现在出版的有些书不适合于孩子。
4）The small
portable battery-operated units are suitable for field
work.
这种用电池作电源的小型便携式仪器适合于野外工作。
appropriate指适合于某一特定场合的要求，接介词
for；强调针对性时用介词 to。
1）He can
make appropriate speech appropriate for every occasion, but he
never
says anything interesting.
针对不同的场合他会讲适当的话，但是他从不说任何有趣的事情。
2）Since you
don’t know them very well, I don’t think it appropriate for you to give her a valuable
present.
既然你跟他不太熟，我认为你送给贵重的礼物不合适。
3）A good
teacher should establish his personal teaching style appropriate to his own teaching
practice.
一个优秀老师应建立适合于自己的教学实践个人教学风格。
4）Preventive
actions shall be appropriate to the effects of the potential problems.
预防措施应与潜在问题的影响程度相适应。
5）I really
don't feel it is appropriate to continue the discussion in such
a large forum.
我真的认为，以这样一种大规模座谈会的方式来继续讨论是不适当的。
proper指对某种情况适当，含适如其分的意思，不少也不为过。
1）The
mother told her younger son that his behavior was not proper for
the occasion.
母亲告知她的小儿子说，在这种场合他的行为是不适当的。
2）If prompt
and proper treatment is given, the patient will soon recover
from is illness.
如果给给予及时适当的治疗,病人很快就会康复。
3）Her good
health is chiefly due to proper diet and regular
exercise.
她身体很好，主要原因是饮食得当以及经常运动。
4）It is not
proper to visit a friend at this late hour, I am afraid.
天这么晚了，去看朋友恐怕不太合适。