1.根据but后面的
2.根据前面的
主谓一致(Agreement of Subject and Verb)是指谓语动词必须同主语的人称和数保持一致，它一直是大学英语四级考试的考查重点。主谓一致原则看起来很简单，但实际运用情况却很复杂。本文对四级考试中经常考查的主谓一致关系的一些特殊规律，做一简要归纳。
一、形式上为复数，但意义上为单数的名词作主语时，其谓语动词通常用单数形式。这类名词有以-s结尾的表示学科、专业、疾病等术语的名词及其他名词。如：economics，linguistics，politics，mathematics，physics，measles(麻疹)，news，works，brains(智慧)等等。例如：
Physics is a difficult subject.
Brains is very important to a person.
二、表示时间、距离、重量、价格、度量衡的复数名词或短语作主语表示一个整体概念时，谓语一般用单数。例如：
Five hours is a short time for such a difficult job.
Ten miles is a long way to walk.
三、集合名词people,police,cattle等作主语时，谓语动词用复数；集合名词army,class,club,committee, crowd,crew,family,gang,herd,jury,public,congress,assembly,band等作主语时，如指整体，谓语动词用单数；如指成员，谓语动词用复数。例如：
The family is going to move to Beijing.
The family have different opinions about their going abroad.
四、分数或百分数及all,part,some,most,half,the rest of等表示部分的词或短语作主语时，谓语动词的单复数与of后的名词一致。例如：
Most of the university students have part-time jobs to finance their tuition.
Most of the work hasn't been finished yet.
五、名词或代词后跟with,along with,like,accompanied by,except,besides,as well as,together with,in addition to,including,no less than,rather than,as much as等引起的结构时，谓语动词随结构前的名词或代词的形式而定。例如：
The president,accompanied by five officials,has gone to USA for a very important meeting.
You，rather than your sister,are responsible for the matter.
六、由and连接的名词的主谓一致情况分以下几种：
1.当and连接的两个名词指同一人、同一事或同一概念时，and后面那个名词前没有冠词，谓语动词用单数。例如：
The teacher and writer is respected by all the people around.
2.当and连接两个都有冠词的名词作主语时，谓语动词用复数。例如：
The teacher and the writer are respected by all the people around.
3.形容词＋and＋形容词＋名词作主语时，谓语动词用复数。例如：
Social and political freedom are limited there.
4.用and连接的单数名词，前面有each,every, many a,no修饰语时，谓语要用单数。例如：
Every man，woman,and child is entitled to take part in the activity.
Each pen and each paper is found in its place.
七、不定式短语、动名词或从句作主语时，谓语动词一般用单数。例如：
Reading in the morning is good for learning English.
但是，当and连接上述两个相同的结构时，谓语动词用复数。例如：
Reading books and playing table tennis are my great pleasure.
八、主语由连词or,either...or,neither...nor,not only...but(also)等连接的两个名词或代词组成时，其谓语动词应与最近的名词一致。例如：
Either you or I am wrong.
Not only his friends,but also he himself is looking forward to the holding of the party.
九、"one or two＋名词复数","one or more＋名词复数"作主语，谓语动词用复数。"one/a＋单数名词or two"作主语，谓语动词可用单数也可用复数。例如：
One or two books are needed.
One book or two is /are needed.
参考资料：http://edu.sina.com.cn/en/2004-03-02/19343.html