Register means how formal or appropriate is the chosen word.
In all languages some words are more formal than others – some words are used on
special occasions, in special contexts and for special reasons, while others words are more common. In English a general rule is that a longer word is more formal than a shorter word.
For example, Phrasal Verbs (a simple verb with a preposition – “to look into”, “to look after”) are the least formal choice and are commonly used in informal spoken
language, notes, emails and postcards (informal writing). However, unusual words,
borrowed into English from other languages, especially Latin and Greek, are quite
formal, or high register.
比如，诸如“to look into”, “to look after”之类的动词与介词组合叫短语动词，是最通俗的用法，常见于非正式的日常对话、短信、邮件和明信片。一般来说，不常用的词汇，外来词汇，特别是拉丁语和希腊语，通常更正式，更讲究一些。
Greek and Latin borrowings into English often have several syllables. These words
are used often in technical, scientific, medical, psychological and philosophical
Which word is easiest to remember?
The smaller or shorter word is the easiest to learn and remember. So, you will
probably learn “look into” or “study” first. Then you will pick up “research” and lastly
“analyse” and “investigate”.
短小的词汇更容易识记，所以呐，学会了look into 和study，然后再弄懂research，最后搞定analyse和investigate。（注意使用方式的不同喔）
1、You will need more reasons and evidence ____ me.
2、These diagrams will need to be accompanied by short ____.
3、There were ____ speaking more women than men in the workforce in the 90s than in previous decades.