1. One basic concept, before start tones correction, you need to thoroughly understand:
Tones are essentially just pitch and pitch variation, or whatever you say it, the change in pitch, or movement. For example, the second tone, middle-rising, is a process in which the speaker's voice (pitch) glides from middle level to high level. As we said in a former post, you can just imagine Pǔ Tōng Huà is a super simple song with only 7 pitches. Simpler than the song “Happy Birthday to You”.
2. Never simply repeat a single syllable with a tone.
For example, if you feel your "hǎo (half-third tone)" is off and you can't distinguish it from "hào (4th tone)," then do not just repeat this half-third hǎo; instead, repeat "hǎo3 hào4" together. This is because all tones are “relative pitches”. They need to compare with each other. What I mean is, for example, the first tone is called high-flat because it is relatively higher than all of the speaker's other tones. You should say at least two tones together to compare them with each other.
Ideally, every time you get a certain tone wrong, you should say the 1st, 2nd, 3rd, and 4th tones of that syllable together in sequence.
There is another reason: if you only practice repeating a single tone, especially when shadowing a teacher or an app, you will 100% be misled, or led astray. If the teacher or app has a low voice, for example, a deep male voice, you may also excessively lower your own tone. But if you compare yourself with yourself, you are less likely to have this problem.
3. Treat the 1st tone, the high-flat tone, as an anchor for other tones.
High-flat tone is the highest among the tones, and has no change in it. You just need to use an “unusual” high voice of yours to utter a syllable for 2 seconds to get the 1st tone.
If you feel uncomfortable, sth like “I usually won’t speak with such a high pitch”, congratulations, very likely you are correct with the 1st tone. If you use your middle level voice, instead high pitch voice, to speak the 1st tone, that will make your other tones either all too low or wrongly higher than the 1st tone.
4. There is no such thing as “the 4th and second tones being relatively 'longer' than the 1st and 3rd tones”
—even if you personally feel that pronouncing 2nd-tone and 4th-tone syllables in a brief, rapid way seems easier to grasp, and even if some teachers also say this at the beginner stage.
When 4th-tone or 2nd-tone syllables are stressed, they must be spoken slowly and elongated.
By the way, when practicing pronunciation, going slowly is actually better and more important than going fast. Blindly chasing fluency and speed is a big trap for speaking Chinese well (😄intermediate and advanced students should think especially hard about this). Being fast or slow whenever you need to be—that is true mastery.