When starting to learn Chinese, a common complaint is that Mandarin syllables and words all sound similar. This is because you haven't grown used to the sounds in Mandarin yet, so it's a matter of not being familiar with the differences between sounds that sound the same to you. This happens when you learn any new language, so Mandarin is not unique.
However, it does happen a lot more in Mandarin and there are several reasons for this, some of which aren't just based on your inability to distinguish sound and tones, but that are objectively difficult.
This article will talk about 3 types of factors that often make Mandarin syllables and words sound similar or identical.
Tones in Mandarin Chinese 汉语的声调
Mandarin is a tonal language, which means that the thepitch of the syllable changes the meaning of the word. Therefore, many distinctions in Mandarin are in a realm you might be completely ignorant of if you come from a non-tonal background, such as English.
Changing the way you pronounce a syllable changes the fundamental meaning of what you're saying. In English, changing pitch is the difference between asking if you want tea by raising your pitch towards the end, or by falling at the end, making it a answer. In Mandarin, that would change the meaning of the word itself.
Limited number of syllables 音节数量有限
Mandarin has a very limited amount of syllables, and there are around 1,000 commonly used syllables.
This includes tones. Compare this with English, which has something closer to 10,000 syllables! The reason is that Mandarin syllable structure is very limited. For instance, there are no consonant clusters and the syllables can only end with vowels or the sounds -n, -ng and -r.
Series of unfamiliar sounds 一系列不熟悉的发音
To add insult to injury, Mandarin has three series of fairly difficult sounds: z/c/s, zh/ch/sh and j/q/x. While some of these sounds exist in English too, most do not.
It should also be mentioned that some native speakers, especially from southern China, don't make a clear difference between these series.
If you think that Mandarin syllables and word sound very similar, perhaps you can find some consolation in the fact that this does go away with practice; you can learn how to distinguish these sounds from each other.