'Could' is one of the most commonly known modal verbs. It is used to show past abilities, possibilities, requests, etc. To put it simply, 'could' is the past tense of modal verb 'can.' For instance:

'Will' is a modal verb that mainly talks about future events. It is used to talk about future plans, habits, assumptions, and predictions. For example:

We use 'could' and 'will' to make offers. Offers are statements that indicate our willingness to do something for someone.
'Could' is more formal than 'will.' Both modals are not definite offers, meaning that we are uncertain if the listener would accept our offer or not.

We also use 'will' and 'could' to ask for favors or ask someone to do something for us at the present time. Requests that are made with 'could' are more formal.

We can use modal verbs to create negative sentences and questions.
When we want to make a sentence negative, we add 'not' to the modal verb as illustrated below:

Take a look at the following examples for clarity:

questions are created through the inversion of the modals. Watch:

'Will' is used to make predictions about the future. These predictions might become true or they may fail to happen. This future may be close or years away. For example:

We use 'could' to talk about past abilities, skills, and talents. These abilities and skills were used in the past and perhaps we stopped using them or we lost them. Sometimes the person who had the ability or talent may be deceased. Have a look:

We use the modal verb 'will' to express that we are ready to do something. For instance:

As shown in the table below, 'could' and 'will' are used as conditional verbs. conditionals are used to show that the occurrence of an event depends on another event or action to happen.

The first type of conditional is used to talk about real situations with a high chance of becoming real. 'Will' and 'could' are both used in this type. Have a look:

Conditional type 2 talks about hypothetical situations in the present and future that are imaginary and perhaps will never come true and their chance of occurrence are low. We use 'could' in this type and we cannot use 'will.' For example:

The third type of conditional talks about an imaginary past and what could be. This past could have happened but it never did and we are just imagining a different past. We use 'could' in this type. Consider the following examples: