وَسُيِّرَتِ ٱلْجِبَالُ فَكَانَتْ سَرَابًا
Sentence structure
This statement parallels the preceding ayah. ٱلْجِبَالُ is the passive subject of سُيِّرَتْ, and the clause فَكَانَتْ سَرَابًا states that the mountains will then become a mirage.
وَسُيِّرَتِ — and the mountains will be moved
Grammar: وَ is a conjunction. سُيِّرَتْ is a form II, third-person feminine singular perfect passive verb; the feminine singular agrees with a non-human plural.
ٱلْجِبَالُ — the mountains
Grammar: A definite nominative broken plural and the passive subject نائب فاعل of سُيِّرَتْ.
فَكَانَتْ — then they will become
Grammar: فَـ marks the resulting state. كَانَتْ is a feminine singular perfect deficient verb whose understood noun هِيَ refers to the mountains collectively.
سَرَابًا — a mirage
Grammar: An indefinite accusative masculine noun and the predicate of كَانَتْ.