Many German verbs combine a base verb with a prefix, and the prefix can be either separable or inseparable. A separable verb (trennbares Verb) splits its prefix from the stem in the present tense, and the prefix moves to the end of the clause. The verb aufstehen (to get up) is a clear example: Ich stehe um 7 Uhr auf means I get up at 7 o'clock, where the conjugated form stehe stands in second position and the prefix auf moves to the end of the sentence.
Common separable prefixes include auf- (as in aufstehen and aufhören), ein- (as in einkaufen and einladen), an- (as in anfangen), mit- (as in mitnehmen), zurück- (as in zurückkommen), aus- (as in ausgehen), and fern- (as in fernsehen). Each behaves the same way in the present tense: the stem is conjugated, and the prefix goes to the end. For example, Ich kaufe im Supermarkt ein means I shop at the supermarket, Der Film fängt um 8 Uhr an means The movie starts at 8, Er hört um 17 Uhr auf means He stops at 5 PM, Nimmst du den Regenschirm mit means Are you taking the umbrella along, Ich komme morgen zurück means I am coming back tomorrow, Wir gehen heute Abend aus means We are going out tonight, Sie lädt ihre Freunde ein means She invites her friends, and Er sieht abends fern means He watches TV in the evening. Some separable verbs also undergo a vowel change in the du and er/sie/es forms, as seen with einladen becoming lädt.
Inseparable verbs (nicht trennbare Verben) keep their prefix attached to the stem in all tenses, and they never add the ge- prefix when forming the Perfekt. The eight inseparable prefixes are be-, emp-, ent-, er-, ge-, miss-, ver-, and zer-. The verb verstehen (to understand) stays together as a single unit, so Ich verstehe nicht means I do not understand. In the Perfekt, bekommen becomes bekommen without any ge-, so Ich habe ein Geschenk bekommen means I received a gift. Similarly, erzählen becomes erzählt (Sie hat eine Geschichte erzählt — She told a story) and vergessen becomes vergessen (Ich habe meinen Schlüssel vergessen — I forgot my key). The rule is simple: if the prefix is one of the inseparable prefixes, do not separate it in any tense and do not add ge- in the Perfekt.