Si le steak est trop sec, tu peux mettre un peu de moutarde à côté.

Si means if here. It introduces a condition:

• Si le steak est trop sec = If the steak is too dry

This is the normal word for if in French when you are talking about a condition.

French often uses the present tense after si for a real or possible situation:

• Si le steak est trop sec, tu peux...

This is similar to English If the steak is too dry, you can...
French does not use the future after si in this kind of sentence.

So:

• Si le steak est trop sec... = correct
• Si le steak sera trop sec... = incorrect in this context

Le steak means the steak, and here it refers to the steak being served or the steak already being talked about.

French often uses the definite article when both speakers know what specific thing is meant.

• le steak = the steak on your plate / the steak in question
• un steak = a steak, any steak, or one steak not yet identified

Here, est comes from the verb être and directly links the subject to the adjective:

• le steak est trop sec = the steak is too dry

You use c’est more often with nouns or for identification:

• C’est un steak = It’s a steak

But with a normal noun + adjective structure, French usually uses:

• Le steak est sec
• La viande est bonne

So le steak est trop sec is the natural form.

Trop means too, and sec means dry.

So:

• trop sec = too dry

Also, sec agrees with the noun:

• le steak est sec → masculine singular
• la viande est sèche → feminine singular

Since steak is masculine, the adjective stays sec.

After a conjugated verb like pouvoir (can / to be able to), the next verb stays in the infinitive.

So:

• tu peux mettre = you can put
• not tu peux mets

This is exactly like English:

• you can put
• not you can puts

Mettre literally means to put, but in food contexts it can also mean to add.

So here mettre un peu de moutarde à côté means something like:

• put a little mustard on the side
• add a little mustard on the side

French uses mettre very widely in everyday situations.

After an expression of quantity like un peu de (a little), French normally uses just de:

• un peu de moutarde
• beaucoup de pain
• trop de sel

So even though moutarde is normally an uncountable noun that might take a partitive article in other contexts, after un peu de, you use de.

Compare:

• Je veux de la moutarde. = I want some mustard.
• Je veux un peu de moutarde. = I want a little mustard.

À côté literally means at the side or next to it, but with food it often means on the side.

So:

• mettre un peu de moutarde à côté = put a little mustard on the side

This suggests the mustard is not mixed into or spread directly on the steak, but served separately beside it.

Not exactly. The meaning changes.

• avec de la moutarde = with mustard
  This just means the steak comes with mustard in some way.
• à côté = on the side
  This specifically suggests the mustard is separate.

So if you want the idea of serving mustard separately, à côté is the better choice.

Tu is the informal singular you. It is used with friends, family, children, or in casual instructions.

• tu peux = informal you can
• vous pouvez = formal or plural you can

So the sentence could become more formal as:

• Si le steak est trop sec, vous pouvez mettre un peu de moutarde à côté.

The grammar stays the same; only the level of formality changes.

Not exactly. It is a suggestion or permission:

• tu peux mettre... = you can put...

It sounds softer than a direct command.

Compare:

• Tu peux mettre un peu de moutarde à côté. = You can put a little mustard on the side.
• Mets un peu de moutarde à côté. = Put a little mustard on the side.

The first is more gentle and helpful.

Yes. Since moutarde is introduced with de, you would usually replace it with en.

For example:

• Tu peux mettre un peu de moutarde à côté.
• Tu peux en mettre un peu à côté.

Here en means some / some of it.

This is very common in French with quantities:

• un peu de moutarde → en
• beaucoup de pain → en
• pas de sucre → en

French often places short location expressions like à côté after the main object or after the verb phrase.

So this order is natural:

• tu peux mettre un peu de moutarde à côté

It sounds more awkward to move à côté earlier unless you are changing the emphasis. The sentence as written is the most neutral, everyday word order.