ماذا أفعل إذا كنت مأمومًا ولم أتمكن من إكمال قراءة الفاتحة أو التشهد الأول قبل أن يركع الإمام أو يقوم؟

The predominant view is that the ma'mum (follower) may lag behind the imam (leader) to complete the Fatihah if the imam bows before the follower finishes it, and the follower is excused in this. Similarly, the predominant view is that the ma'mum should complete the first tashahhud (testification) if the imam stands for the third unit of prayer before the ma'mum completes it, and then the ma'mum follows him. This is especially true if the first tashahhud is deemed obligatory, and lagging behind to complete it does not cause harm.