1. Primary halide for instance CH3Cl can undergo only substitution.

Reason: For elimination the halide should have a hydrogen at the next carbon ( beta-elimination).  Also substitution occurs through exclusively by S_N2 mechanism (cannot form a stable carbocation).

2. A primary halide of the type RCH₂CH₂Cl will undergo mainly S_N2, with a hindered base it will be mainly E2.

Reason: cannot form a stable carbocation so no S_N1. The hindered base will not be able to behave as a nucleophile so substitution is not favoured.

3 .A secondary halide behaves like a primary halide, but depends on the reaction conditions.

Reason: as in 2.

4. A tertiary halide: no S_N2, strong base E2 predominates

Reason: forms a stable carbocation, also there is steric hindrance for the nucleophile to reach the carbon bearing the leaving group. E2 predominates because of the use of hindered base.

5. Higher temperatures favour elimination.

Reason: Elimination reactions have higher activation energy barriers, because there are more number of bonds broken in elimination compared to substitution.
Higher temperature provides more energy to overcome this energy barrier hence elimination is favoured.