I have a Suzuki Swift petrol 2008. car that has sometimes struggles in first and second gear after stopping at junctions but other times it is fine what does this mean ?
If you consider that it is necessary to fully depress the clutch to engage first gear from neutral, and allowing for your consistent practice of full pedal activation, one might suggest that the clutch is not being fully disengaged. If your clutch mechanism is hydraulic, and air has entered the system, you can have the problem you describe. Other problems with the hydraulic clutch system can produce similar results.
If it is a cable activated clutch, a misadjustment will result in a similar problem.
You can test this to a fashion by finding a flat surface on which to park. Ensure that the transmission is in neutral and that the vehicle does not roll with the handbrake off.
With the engine off, engage first gear, keeping the clutch pedal depressed. Upon starting, leave the pedal at the full down position. If the vehicle rolls, this is because the clutch disk is partially engaging the flywheel, providing power to the transmission. The clutch is also slipping more than engaging, increasing wear on the disk and flywheel surfaces.