There are currently two forms of Chinese writing: the simplified (used in mainland China and Singapore), and the traditional (used in Taiwan, Hong Kong, Macao).
After taking power in 1949, the Chinese Communist government started a reform in order to simplify the sinograms to facilitate their learning and to create a phonetic alphabet based on Latin letters, the pinyin.
Out of more than 50,000 existing characters, 2,236 have been simplified. It is therefore important to keep in mind that the existence of a traditional "double" concerns only a small part of the chinese characters.
50,000 characters is a scary number. But you should know that Chinese people commonly know between 3000 and 4000 characters. The history of Chinese writing is very long, more than 3000 years. The literary wealth is impressive. Many characters are no longer used. With the 900 most common Chinese characters, one can read 90% of the characters in a newspaper. This figure rises to 97.97% for 2500 characters and to 99.48% for 3500 characters.
This website's courses are based on simplified Chinese characters. Traditional Chinese characters are provided as an option for those who want to learn both forms of writing. Thoses courses have been designed to optimize this learning as much as possible. The didactic principle is to present traditional Chinese characters as a logical extension of the course that are integrated into the progression in order to avoid having to relearn everything. The lessons and exercises are constructed in such a way as to allow you to acquire the ability to switch from one writing to another easily.
To optimize the learning of traditional characters, you should keep in mind that the simplification follow some rules. For a learner of Chinese, it is interesting to know 3 of them:
1 - If one component of a Chinese character is simplified, all the characters that share this component will use the same simplified component. For example, we saw the Chinese character 吗 which is composed of 口 the mouth and 马 the horse. The "horse" component 马 is a simplification of 馬. Thus, all characters containing the component "horse" 馬 will follow the same simplification as this component:
媽 ⟶ 妈 螞 ⟶ 蚂 碼 ⟶ 码 嗎 ⟶ 吗   etc.
The same applies to for the character 见 (see) which is the simplification of 見 and which we find as a component in some characters:
現 ⟶ 现 視 ⟶ 视 寬 ⟶ 宽   etc.
This rule helps learn a large number of simplified ⇆ traditional pairs and greatly accelerates the learning progression of traditional Chinese writing.
2- The second rule of simplification which is interesting for the learner is the replacement of the phonetic part of a Chinese character. Some Chinese characters, called "ideophonogram" , are composed of a part indicating the idea and a part giving an indication of pronunciation (sometimes not obvious given the evolution of the Chinese pronunciation). For example, the character 種 zhòng "to plant" is composed on the left side of the element 禾 "cereal" which gives the meaning, and on the right side of 重 zhòng (heavy) which gives the indication of pronunciation. This component has been changed to 中 which is pronounced zhòng (hitting a target) or zhōng (middle).
This second rule is less obvious than the first, because it is less systematic. However, logic remains a great help for the Chinese learner who wishes to study simplified and traditional writing at the same time.
3- Elimination of one or more components. For example the character 開 "open" has been simplified by truncating the component 門 "door" to keep only 开, which is easier to write. This simplification rule is easier when you first learn traditional Chinese characters and then the simplified ones. However, it is still very useful for the learner, even if he begins by learning the simplified Chinese characters. The remaining component gives an indication and facilitates memorization.
Finally, there are other simplification rules, but they do not help the learner. We will not talk about them so as not to overload the course with unnecessary information.