Modal auxiliaries and their replacement forms

Most grammatical problems are either known to be difficult (e.g. participle constructions) or have hellishly frightening names (conditional complex sentences). With modals things are totally different, because here the two most important questions are...

1. What's the problem?

and then

2. What the hell is a modal?

Like always, we start with the good news, which is also the answer to the second question. In this case this is the fact that the number of modals is limited. (The list in the picture is either too long or too short, depending on how you see things, but more on that later).

Here they are. modals

Of course you can easily say sentences like

I learn English.

However, if you want to express the truth (the whole truth and nothing but the truth) you will have to admit that one more word is needed to describe the situation correctly:

I must learn English.

Can you see the difference? It may be difficult and you might need to concentrate, will you? Yes, you guessed it, all words in italics are to be dealt with in this modals section, either as part of the rule or as exceptions.

Do you know now what a modal does? In simple words, it somehow modifies the mode of a statement. (I know that mnemonic is somewhat clumsy. Send me yours if it is better.) So the aspect of the statement that is changed by the modal is called mode. Our sentence above

I must learn English.

says that learning this language is nothing you chose to do on your own free will (nobody is blaming you), but it expresses the mode of obligation, while an exchanged modal auxiliary, like in

I can learn English.

will probably mean that you are able to learn English; it expresses ability, or it could mean that you have the chance to learn English and thus express possibility. What exactly the sentence says will always depend on the context.
This is what modals do: They assign a modal concept to a statement.

But, you guessed it again, there is a catch. Modal auxiliaries do not only offer possibilities, but they also mean trouble: Modal auxiliaries are so-called defective verbs. They have an incomplete (defective) grammatical paradigm. They normally

'All right', you may say in all your impressive cleverness, 'so I use modal concepts in present tense only'. But how can you tell all those interesting stories about yourself, then?

You want to touch the heart of this breathtaking girl (well, we both know it's not literally her "heart" we are speaking of) by telling the story of your sad youth and you reveal to her
"When I was only little I ... learn English."
The signs of her sympathy will remain modest and she will continue playing with her mobile unless you learn to express obligation in the past.

Because must cannot be used except in present tense, we need some replacement; an analogue expressing the same mode without having the same limitations. The form have to fulfils all these requirements. So your heart-breaking story would sound like this:
"When I was only little I had to learn English."
Right after this, the girl's eyes will become wet with tears of sympathy and she will take you in her arms and her mouth near your ear will form the words ... STOP DAYDREAMING! THIS IS A GRAMMAR SITE!

In case you haven't noticed: You just read the answer to question number one at the top of the page. The following table gives an overview of all modals, the possible modal concepts they represent and their analogues. Hover on the check marks with your mouse pointer for an example sentence. Check marks without brackets indicate the concept most frequently expressed by the modal auxiliary. Click on the modals themselves for more detailed information. (And this is where the real work begins).

One more note: If you are wondering, where your favourites would, should and could have gone... You will find them under will, shall and can, respectively.

About using the analogue forms

The phrases in the Analogue column always refer to the modal's primary concept. So just assume, you want to use can (like in I can do it.) in past tense. You would replace can with be able to and then shift this into simple past. The result will be I was able to do it. Doesn't it feel great to talk about your abilities in so much detail?

Will is an exception, because there is no primary concept. Click on it to read the details. Like and need are special, too, because they are no real auxiliaries. Somehow they are full verbs in modals' clothing. Just click and learn.

In addition to the real modal auxiliaries,...

...there are some more words and phrases which express modal concepts, but aren't modal auxiliaries. These are: ought to, had better, dare and used to.

Here is what they do: