(1) relax是“松弛，放松，轻松”，用于“紧张、严格性、控制、警惕”等，可作及物动词，也可作不及物动词。
- Now let's stop working and relax for a moment.
现在咱们停下工作，放松一会儿吧。 - The government has never relaxed its hold on prices.
政府从未放松对物价的控制。 - We should never relax our vigilance in face of the threat of aggression.
面对侵略的威胁我们永远不应放松警惕。 - You should relax a month or so before going back to work.
在回去上班前，你应该休息上一个月。
(2) loose多作形容词，也可作动词。作动词时，表示“使无约束，使松弛，放开，松开，解开”。
- He loosed the birds from the cage.
他把鸟从笼中放飞了。 - Can you help me to loose the knot?
你能帮我解开这个结吗？ - Wine loosed his tongue.
酒使他说话多起来。
(3) loosen与loose用作动词时，意思相同，但loosen更常用。
- Loosen the screw a bit.
把螺丝拧松一点。 - This medicine may loosen your cough.
这药能缓解你的咳嗽。
＊其他用法还有loosen one's belt, loosen (up) one's muscles, loosen a stone in a wall, loosen a tie/the soil/one's coat/discipline等。
(4) slack多作形容词，表示“松，不严格，松弛，懒散，马虎，萧条”等，有时也作动词。用作及物动词时，表示“放松(绳子等)”；作不及物动词时，表示“懈怠，疏忽，懒散，放慢，放松(努力等)”。
- Slack the rope before trying to untie the knot.
在试图解这结之前，把绳子放松。 - People usually slack off towards the end of a hard day's work.
人们在一天的努力工作临近结束时，往往会懒散下来。 - The train slacked off (its speed) as it came into the station.
火车在进站时放慢了速度。
(5) ①slacken意思是“(使)缓慢，迟滞，变得不景气”；“(使)松弛，放松，(人)变得懒散”。
- Slacken speed a little, please.
请放慢点速度。 - Don't slacken the reins or the horse may bolt.
不要放松缰绳，否则马会惊跑的。 - Don't slacken your efforts when the job is only half done.
工作只做了一半时不要放松努力。 - Work slackens on a hot day.
天气炎热，工作就会迟滞不前。 - The students tend to slacken off after the first few weeks when their interest is new. (也可用ease off/up)
在最初几周学生们对什么都感到新鲜之后，往往会懒散下来。 - Slacken off those ropes there, there's a storm coming.(也可用slack away或slacken away)
把那些绳子放松，要来暴风雨了。 - House sales in this city usually slacken off during our severe winters.
在严冬时节，这个城市的房屋销售通常不景气。
②slacken up意为“降低速度或努力”；可以带宾语，也可不带。
- The doctor advised him to slacken up for a few months, as his lifestyle was having a bad effect on his heart.
医生建议他放松上几个月，因为他的生活方式对他的心脏产生了不利影响。 - Please slacken up your speed, I can't run as fast as you!
请放慢一下速度，我跑不了你那么快！
＊以上两句也可用slack up。
