Annem bahçeyi çiçeklerle süslüyor.

In Turkish, family‐member nouns almost always carry a possessive suffix rather than a separate pronoun.
• Annem = “my mother.”
• Saying Benim annem is grammatically correct but sounds redundant or emphatic.

Because it’s a definite, specific direct object, it takes the accusative suffix -i / -ı / -u / -ü (here -yi after a vowel).
• bahçe (garden) → bahçeyi = “the garden” (that is being decorated).

That’s the instrumental case suffix meaning “with” or “by means of.”
• çiçek (flower) → çiçekle = “with a flower.”
• çiçekler (flowers) + -le → çiçeklerle = “with flowers.”

Because the action uses multiple flowers.
• çiçekle = “with a flower” (one)
• çiçeklerle = “with flowers” (many)

1. Start with the infinitive süslemek (“to decorate”).
2. Remove -mek, giving the stem süsl- (note: many two-syllable -mek verbs drop the -e-).
3. Add the present‐continuous suffix -üyor (vowel harmony for ü).
4. No extra ending for 3rd person singular.
   Result: süsl- + üyor = süslüyor (“is decorating”).

Yes. Turkish allows flexible word order for emphasis:
• Çiçeklerle annem bahçeyi süslüyor
(Emphasizes “with flowers”.)
• Bahçeyi çiçeklerle annem süslüyor
(Emphasizes “my mother”.)
The default is S–O–V, but adverbials or instruments can move before the verb or even to the front.

Yes. Turkish typically omits the subject if it’s clear from context.
• Bahçeyi çiçeklerle süslüyor still means “She is decorating the garden with flowers,” but you need context to know who “she” is.