«أَخَذْتُ مَا أَعْطَيْتَنِي» — I took what you gave me: relative «مَا» as object, with an omitted returning object of «أَعْطَيْتَ».
«مَا هَذَا؟» — What is this? Here «مَا» is interrogative, not relative.
Distinguish a question from a relative construction with a ṣilah.
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Relative Nouns — Ṣilah, ʿĀʾid, Forms and Syntax
«أَخَذْتُ مَا أَعْطَيْتَنِي» — I took what you gave me: relative «مَا» as object, with an omitted returning object of «أَعْطَيْتَ».
«مَا هَذَا؟» — What is this? Here «مَا» is interrogative, not relative.
Distinguish a question from a relative construction with a ṣilah.