Мох растёт под деревом.

Because the preposition под normally takes the instrumental case when it means location, as in under.

• дерево = nominative singular, the dictionary form
• деревом = instrumental singular

So под деревом literally means under a tree / under the tree.

Not always. Под can take different cases depending on the meaning:

• под + instrumental = location
  
  • под деревом = under the tree
• под + accusative = movement toward/under something
  
  • под дерево = to under the tree / underneath the tree

In this sentence, there is no movement. The moss is simply located there, so Russian uses instrumental.

Because the verb is расти (to grow), and its 3rd person singular form is растёт.

The ё shows both the correct vowel and the stress:

• растёт = pronounced roughly ras-TYOT

In normal printed Russian, ё is sometimes written as е, so you may also see растет, but it is still pronounced растёт.

Растёт is:

• present tense
• 3rd person singular
• from the infinitive расти (to grow)

So it means:

• it grows
• is growing

Since мох is singular, the verb is singular too.

Russian has no articles, so there is no direct equivalent of a/an or the.

That means под деревом can mean:

• under a tree
• under the tree

The exact meaning depends on context.

Yes. Мох (moss) is a masculine noun.

You can tell from the basic form and from the verb agreement:

• мох = singular masculine noun
• растёт = singular verb form matching the singular subject

In the past tense, the masculine gender would show more clearly:

• Мох рос. = The moss grew.

Yes, Russian word order is fairly flexible.

The neutral, straightforward order here is:

• Мох растёт под деревом.

But other orders are possible, for example:

• Под деревом растёт мох. = Under the tree, moss grows.
• Растёт мох под деревом. = less neutral, more stylistic or context-driven

Russian word order often changes for emphasis, topic, or style, not because of strict grammar rules like in English.

Мох is pronounced approximately like mokh, with a hard kh sound at the end.

That final consonant is х, which is not like English h in all positions. It is a throat sound, similar to:

• the ch in Scottish loch
• or German Bach

So:

• мох ≈ mokh

Because дерево is a neuter noun ending in -о, and in the instrumental singular, many such nouns change:

• -о → -ом

So:

• дерево → деревом
• compare:
  • окно → окном
  • место → местом

This is a very common instrumental singular pattern for neuter nouns.

Yes. Russian present tense often covers both meanings that English separates.

So Мох растёт под деревом could mean:

• Moss grows under the tree
• The moss is growing under the tree

Context tells you whether it is a general fact or something happening right now.

Yes, both singular and plural are possible, depending on what you mean.

• Мох растёт под деревом. = Moss grows / The moss is growing under the tree.
• Мхи растут под деревом. = Mosses grow under the tree.

Russian often uses the singular for material or mass-type meanings, much like English moss. So the singular here sounds very natural.