Si, tant and tellement are adverbs that express intensity. Thanks to them, you will be able to emphasize so much what is important in your sentence and to bring so many nuances to your speech !
Nonetheless, these three tool words are not used exactly like in English (no worries, nothing so complicated!). Let's see how to used them properly step by step !
Si is used before an adjective or an adverb. It is the equivalent of so in English.
Il fait si chaud que nous avons mis la climatisation. (It feels so hot that we switched the A/C on.)Elle a chanté si bien que le public était émerveillé. (She sang so well that the public was astonished)* Note that the buffer word que is mandatory in French while the buffer word that is optional in English. Another small difference you have to keep in mind when you make a complex sentence in French.
Tant is placed after a verb.
Il travaille tant qu’il est épuisé. (He works so much that he is exhausted.)Vous parlez tant que vous ne vous écoutez plus. (You speak so much that you no longer listen to each other.)Be careful ! When using compound tenses, many tool words are placed between the helping verb and the participle… This is also the case for tant.
Je me suis tant amusé aujourd'hui. (I had so much fun today.)Il a tant plu cette semaine. (It has rained so much this week.)On va tant voyager cet été ! (We will travel so much this summer!)If you want to use tant with a noun, then remember to place it before the noun and to add the small tool word de. (If you wonder why we need this little de, remember this is the main rule for every adverb preceding a noun.)
Il y a tant de monde dans ce magasin ! (There are so many people in this shop!)J'ai tant de mal à faire cet exercice. (I struggle so much with this exercice.)Il y a tant de neige sur cette montagne (There is so much snow on this mountain.)Even if this word is a bit longer to spell than its two other friends, it is way easier to use. So, feel free to use it when ever you don't know which intensity adverb you need.
Tellement can be used with a noun. (But think to put the buffer de before the noun!)
J'ai tellement de chance de te connaître. (I am so lucky to know you.)Tellement can be placed before an adjective.
On a tellement froid ce matin… (We are so cold this morning...)Tellement can accompany another adverb.
Ce taxi nous a regardé tellement bizarrement (This taxi stared at us so weird.)Tellement can even be used alone, as an interjection.
C'était trop bien cette expo ! - Mais tellement ! (This exhibition was so great! - Hell yes!)