schlecht
bad
Of low quality, not meeting expectations, or faulty.
Der Film hatte eine schlechte Bewertung und hat mir auch nicht gefallen.
The film had a bad review, and I didn't like it either.

bad
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bad
Of low quality, not meeting expectations, or faulty.
Der Film hatte eine schlechte Bewertung und hat mir auch nicht gefallen.
The film had a bad review, and I didn't like it either.

To say that you feel unwell or nauseous, you use the dative construction 'Mir ist schlecht' (I feel sick). The sentence 'Ich bin schlecht' means 'I am a bad person' and is a very common mistake.
Ich glaube, ich habe etwas Falsches gegessen, mir ist schlecht.
I think I ate something bad; I feel sick.
'Schlecht' (bad/badly) can be an adjective that describes a noun or an adverb that describes a verb. In German, the word has the same form in both cases when it comes after the verb.
Das ist ein schlechter Witz. (Adjektiv)
That is a bad joke. (Adjective)
The adjective 'schlecht' (bad) is the basis for the verb 'sich verschlechtern' (to get worse), which means 'schlechter werden' (to become worse). It describes a negative process.
Sein Gesundheitszustand hat sich über Nacht verschlechtert.
His state of health worsened overnight.
In the meaning 'of low quality', 'schlecht' (bad) is often combined with nouns like 'Qualität' (quality), 'Arbeit' (work), 'Leistung' (performance), or 'Note' (grade).
Für diese schlechte Arbeit bekommst du kein Geld.
You won't get any money for this bad work.
To describe unfavorable circumstances, 'schlecht' (bad) is often used with 'Wetter' (weather), 'Licht' (light), or 'Zeiten' (times).
Bei dem schlechten Licht kann ich kaum lesen.
In this bad light, I can hardly read.
When 'schlecht' (bad) is used in connection with food or drink, it almost always means 'spoiled'. It is often combined with the verbs 'sein' (to be) or 'werden' (to become).
Die Milch riecht komisch, ich glaube, sie ist schlecht geworden.
The milk smells strange; I think it has gone bad.
good
'gut' and 'schlecht' are a very common pair of opposites. They can evaluate quality, execution, health, morality, or a situation. 'gut' means that something is positive, suitable, or successful. 'schlecht' means that something is not good, has flaws, is unpleasant, or does not fit. Both also work as adverbs: you can work, hear, or speak well or badly. Context is important because both words are used in many areas.

Im alten Büro war das Licht so schlecht, dass alle schnell müde wurden.
In the old office, the light was so bad that everyone quickly got tired.
Nach dem Kurs kann Maria schon gut Deutsch sprechen.
After the course, Maria can already speak German well.