汗马功劳 distinctions won in battle, one's contribution in work
e.g. 双边贸易谈判，他夜以继日地东奔西走，立下了汗马功劳。
During the bilateral trade negotiations, he spared neither day nor night to make them a success. His contribution was indeed great and is highly appreciated.
 
悬崖勒马 pull back before it's too late
e.g.你虽然做过错事，但如能悬崖勒马重新做人，仍然会有光明的前途。
In spite of having done wrong, you can still pull back now before it is too late and work for a bright future.
 
一马当先 be the first, take the lead
e.g.大伙儿去爬山时，他一马当先，往山头冲上去。
When we were climbing the mountain, he took the lead and made for the summit.
 
单枪匹马 do something single-handedly
e.g.中锋小宋常常单枪匹马带球冲到敌阵，避过重重围阻射门，且十有九中。
Xiao Song, the center forward, takes the ball single-handedly into the rival' scourt, dodges all obstacles and shoots, he always manages to score nine times out of ten.
 
老马识途 an old hand is the best guide, an old horse knows the way
e.g.这段山路崎岖难行，老张老马识途，所以让他带路。
The road is rugged and winding, so we have Lao Zhang lead the way because an old hand is the best guide.
 
马到成功 win instant/speedy success
e.g.该研究所设备好，人才济济，若把这个问题交给这个所，必定能够马到成功。
If the problem is given to this research institute, which is well-equipped and well-staffed, we believe it can be both successfully and quickly done.
 
秣马厉兵 be combat-ready, (feed the horses and sharpen the weapons) make active preparations for war
e.g.两支球队明日决赛，双方都在秣马厉兵，争取拿出最好的表现。
The two teams will play in the finals tomorrow. Both sides are combat-ready and hoping to do well.
 
走马观花 give a hurried and cursory glance at, cast only a passing glance at
e.g.由于有公事在身， 他只能走马观花似地看看三亚的风景。
Having business at hand, he only had a passing glance at Sanya.
 
天马行空 an unrestrained and vigorous style of imagination (that brims with talent)
e.g.给孩子一张白纸，他就可以天马行空。
Give your child a piece of blank paper, and he will show you an unrestrained imagination.
 
塞翁失马 a blessing in disguise, a loss may turn out to be gains
e.g.这次峰会的失败或许是塞翁失马。
The failure of the summit may be a blessing in disguise.