Explain the concept of verb tense agreement in conditional sentences using the past tense.

In conditional sentences, verb tense agreement refers to the consistency between the verb tenses used in the main clause and the subordinate clause. When discussing conditional sentences in the past tense, there are three main types: the zero conditional, the first conditional, and the second conditional.

1. Zero Conditional:
The zero conditional is used to express general truths or facts that are always true. In this type of conditional sentence, both the main clause and the subordinate clause are in the present tense. For example:
- If it rains, the ground gets wet.

In this sentence, "rains" and "gets" are both in the present tense, indicating that the result (the ground getting wet) always happens when the condition (rain) is met.

2. First Conditional:
The first conditional is used to express possible or likely future events. In this type of conditional sentence, the main clause is in the future tense (usually with "will" or "shall"), while the subordinate clause is in the present tense. For example:
- If it stops raining, we will go for a walk.

In this sentence, "stops" is in the present tense, indicating the condition that needs to be met, while "will go" is in the future tense, indicating the result that will happen if the condition is fulfilled.

3. Second Conditional:
The second conditional is used to express hypothetical or unlikely events in the present or future. In this type of conditional sentence, the main clause is in the conditional tense (usually with "would" or "could"), while the subordinate clause is in the past tense. For example:
- If I won the lottery, I would buy a house.

In this sentence, "won" is in the past tense, indicating the hypothetical condition, while "would buy" is in the conditional tense, indicating the hypothetical result if the condition were to be fulfilled.

Overall, verb tense agreement in conditional sentences using the past tense involves ensuring that the verb tenses in both the main and subordinate clauses are appropriate for the type of conditional being expressed. The zero conditional uses present tense in both clauses, the first conditional uses future tense in the main clause and present tense in the subordinate clause, and the second conditional uses past tense in the subordinate clause and conditional tense in the main clause.