Use sequence settings that match your source material. If you don't know how, start reading the help file to get acquainted with the program.
As pointed out, the source video (HD) doesn't match the Sequence setting (SD PAL), but if you DO want to edit the HD clips in an SD sequence, go to Preferences and set the checkbox for "Scale to Frame Size", then as you import clips into project they will automatically fit. For clips already imported, you can right-click clip in timeline and check "Scale to Frame" manually.
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