我记得就是说好像上面一行最后一个单词 如果太长了 加入这个单词要分到下面一行 什么情况下需要用-呢？ 还是都需要？
如何区分是因为分行了此案有“-” 还是因为单词本身就有这个符号呢？

你这次问的是一个很大的问题。 连字符（连字号）一直是一个很复杂的符号。它有很多功能。至于你所说的“如何区分”，首先你要知道它本身就是一个单词（根据排版的需要从不同音节连接的地方断掉）还是由两个单词通过连字符构成的一个单词。（如：a 3-year-old boy，这显然是一个连在一起的词。)


连字号（-；英文：hyphen）是西方字母语言里使用的一个标点符号，主要用处是标志合成词，比如，“ice-cream-flavored candy”（冰激凌味道的糖果）。中文一般用于翻译自外语的人名、地名上。

在欧洲字母文字排版技术上，连字号也用在排版过程上。这种排版过程被称为断字（英语：Hyphenation）。以下是在两端对齐排列方式里的断字比例：

不用连字号 使用连字号
We, therefore, the
representatives of the United
States of America...
We, therefore, the represen-
tatives of the United States
of America...


两端对齐通常被报刊使用。可见，不用连字号能造成不美的空格差距。断字技术对词的选择还有要求，通常选择的断字词都是较长的词。如果一个词只有两个字母，把它分段，阅读时就有些别扭。分段地点也有受考虑。通常结果能让读者使用字的前段猜测全字的意思。

连字符最常用于将单个单词分成几个部分，或将几个常用的单词连成一个单词。

连字符没有明确的使用规则。后面都是英文的，稍稍看一下就好了。

A definitive collection of hyphenation rules does not exist. Therefore, the writer or editor should consult a manual of style or dictionary of his or her preference, preferably for the country in which he or she is writing. The rules of style that apply to dashes and hyphens have evolved to support ease of reading in complex constructions; editors often accept deviations from them that will support, rather than hinder, ease of reading. Spaces should not be placed between a hyphen and either of the words it connects except when using a suspended hyphen (e.g. nineteenth- and twentieth-century writers—see below).

The use of the hyphen in English compound nouns and verbs has, in general, been steadily declining. Compounds that might once have been hyphenated are increasingly left with spaces or are combined into one word. The sixth edition of the Shorter Oxford English Dictionary removed the hyphens from 16,000 entries, such as fig-leaf (now fig leaf), pot-belly (now pot belly) and pigeon-hole (now pigeonhole).[1] In other countries hyphens are dropped in favor of connecting the two-word compounds. Use of the hyphen is particularly avoided by those concerned with visual cleanliness, for example writers of advertising copy, packaging labels etc.

However, a significant number of compounds are still routinely hyphenated (e.g. breast-feed, add-on (noun), get-together, merry-go-round). Hyphenation remains the norm in certain compound modifier constructions and, amongst some authors, with certain prefixes (see below). Hyphenation is also routinely used to avoid unsightly spacing in justified texts (for example, in newspaper columns).