In English grammar, first conditional sentences refer to statements that describe a possible future event or condition dependent on a present or future condition. These sentences describe a hypothetical situation and the likely outcome. The first conditional typically takes the form of "if + present simple, will/shall + base form" or "if + present simple, can/could + base form" as an example "If it rains, the streets will be wet" or "If I finish my work on time, I can go to the party."

A first conditional sentence consists of two parts: the condition (if clause) and the result (main clause). The condition is in the present simple, and the result is in the future form, which indicates that the outcome is uncertain; it may or may not happen.

Examples of first conditionals:

• If I study hard, I will pass the exam.
• If she leaves on time, she can catch the train.
• If the weather is nice, we will have a picnic in the park.

It's important to note that, like zero conditionals, the first conditional also describes a cause-and-effect relationship, but this time, the outcome is uncertain; it may or may not happen. First, conditionals also describe hypothetical situations and are used to talk about future possibilities or predictions based on present or future conditions.

The first conditional is also used to express predictions, and future possibilities, the first part of the sentence (if clause) states a possible condition in the present or future, and the second part of the sentence (main clause) states the likely outcome or result of that condition. For example, "If it's sunny tomorrow, I'll go for a walk."

It's also possible to use different verb forms in the main clause, for example: "If I get an invitation, I may go to the party" or "If he makes enough money, he can buy a new car." This usage shows a greater degree of uncertainty about the outcome of the condition.

Additionally, in the if clause, you can use various words or phrases such as: unless, when, as soon as, etc. These phrases can be used interchangeably with it, but the sentence's meaning may change. For example, "If he finishes his work, he'll come to the party." can be rephrased as "When he finishes his work, he'll come to the party" or "As soon as he finishes his work, he'll come to the party".

Lastly, it is worth noting that, like in all conditional sentences, the order of the clauses can be changed depending on the emphasis you want to give. The word "if" could be placed at the beginning or end of the sentence, and it can give different emphasis to the condition or the outcome. "If I finish my work on time, I can go to the party." compared to "I can go to the party if I finish my work on time."