Consider the difference between these two examples: 

The first example is a simple factual statement. The speaker participates in a swimming activity every week on Tuesdays. 

The second example uses the modal verb can. Notice how the meaning changes slightly. The speaker does not swim every Tuesday; they’re saying they are capable of swimming every Tuesday if they need to. It’s hypothetical. 

Modal verbs are quite common in English, and you’ve probably seen them hundreds of times without actually knowing their name. The most frequently used ones are: 

There are more modal verbs, although the ones above are the most common. Some modal verbs are outdated and rarely used—like shall and ought to—while others are more colloquial—such as got to, need to, or have to. Some express very specific conditions that don’t come up often, like dare, for example, “I dare say.” The phrase , as in “I used to be an English student, too,” also behaves like a modal verb. 

You don’t need to come if you don’t want to. 

If I stay in my normal job, I may have been a millionaire – Normal işime devam etseydim şimdiye milyoner olmuştum belki

I could have won the race if I ran much more fast – Daha hızlı koşsaydım yarışmayı kazanabilirdim

They must have been at home last night because their lights were on – Dün gece evdeydiler (tahmin) çünkü ışıkları açıktı

You must have been so tired last night after sport – Dün gece spordan sonra çok yorgun olmuş olmalısın

Jack must have cheated in the exam because he got 100 points even his top grade ise 20 – Jack sınavda kopya çekmiştir çünkü 100 puan aldı (en yüksek notu 20 olmasına rağmen)

She couldn’t have gone because her pursue is on me – O gitmiş olamaz çünkü cüzdanı burada

It couldn’t have rained so much, if it could, you should be wet now – Çok yağmur yağmıyor sanırsam. Aksi taktirde ıslanırdın

He may have had nasal operation because he said he can’t breath comfortably – O burnundan rahat nefes alamıyor. Burun ameliyatı geçirmiş olabilir

You can’t have eaten all food because it was for five people – Tüm yemeği yemiş olamazsın çünkü yemek 5 kişi içindi