Je garde le sachet de riz dans le placard.

Not usually in this sentence. Garder can sometimes mean to guard or to watch, but here it means to keep or to store.

So Je garde le sachet de riz dans le placard means that I keep/store the packet of rice in the cupboard.

This is a very common thing in French: one verb can have several meanings depending on context.

Because je takes the first-person singular form of the verb.

For garder in the present tense:

• je garde
• tu gardes
• il/elle/on garde
• nous gardons
• vous gardez
• ils/elles gardent

So with je, the correct form is garde.

Le is the masculine singular definite article, meaning the.

Le sachet de riz means the packet of rice, not just a packet of rice.

French uses articles more regularly than English, so where English sometimes drops the, French often keeps it.

Because de is normally used after a noun to show content, type, or material.

So:

• un sachet de riz = a packet of rice
• une tasse de thé = a cup of tea
• une bouteille d’eau = a bottle of water

Using du here would sound wrong in standard French, because de is the normal pattern after a container noun like sachet.

Sachet usually means a small bag, packet, or sachet.

Depending on context, it could refer to:

• a packet of rice
• a little bag of sugar
• a tea sachet
• a small pouch

In this sentence, le sachet de riz most naturally means the packet/bag of rice.

Dans means in or inside, so dans le placard means in the cupboard / in the cabinet.

It is used because the rice is located inside the cupboard.

This is the natural choice when something is physically inside something else.

Placard can mean a cupboard, cabinet, or sometimes a closet, depending on context.

In a kitchen or food-storage context, placard is often best understood as:

• cupboard
• kitchen cabinet

So in this sentence, placard is probably where food is stored.

Le placard means the cupboard, while un placard means a cupboard.

French often uses the definite article when the speaker has a specific, known place in mind. For example, if there is a usual cupboard where the rice goes, le placard is natural.

So this sentence suggests:

• either both speaker and listener know which cupboard it is
• or it is the normal cupboard used for that purpose

Yes, it is in the present tense.

Je garde le sachet de riz dans le placard can mean different things depending on context, such as:

• I keep the packet of rice in the cupboard
  (a general habit or usual location)

• I am keeping the packet of rice in the cupboard
  (what I’m doing now, in some contexts)

French present tense often covers both the English simple present and parts of the present progressive.

Yes, it is very similar here.

The structure is:

• Je = subject
• garde = verb
• le sachet de riz = object
• dans le placard = place/location phrase

So the pattern is basically:

Subject + Verb + Object + Location

That makes this sentence fairly straightforward for an English speaker.

Yes, but the meaning changes a little.

• Je garde le sachet de riz dans le placard
  = I keep/store the packet of rice in the cupboard

• Je mets le sachet de riz dans le placard
  = I put the packet of rice in the cupboard
  (focus on the action of placing it there)

• Je range le sachet de riz dans le placard
  = I put away / store the packet of rice in the cupboard
  (focus on tidying or putting it in its proper place)

So garder is about where it is kept, while mettre and ranger are more about the action of placing it there.

A simple pronunciation guide is:

Je garde le sachet de riz dans le placard
≈ zhuh gard luh sa-shay duh ree dahn luh pla-kar

A few helpful notes:

• Je sounds like zhuh
• garde has a hard g
• sachet sounds like sa-shay
• riz sounds like ree
• dans has a nasal vowel, roughly dahn
• the final d in placard is usually silent

There is no major mandatory liaison here that changes the basic pronunciation for a beginner.