Не трогай сосульки: одна сосулька может упасть прямо на тротуар.

Both are possible, but they feel a little different.

• не трогай uses the imperfective verb трогать
• не тронь uses the perfective verb тронуть

In warnings and general prohibitions, Russian very often prefers the imperfective imperative:

• Не кури. = Don’t smoke.
• Не открывай окно. = Don’t open the window.
• Не трогай сосульки. = Don’t touch the icicles.

So не трогай sounds like a normal, natural warning: don’t mess with / don’t touch them.

Не тронь is possible too, but it is often felt as sharper, more abrupt, or more like don’t touch it even once.

Трогай is the informal singular imperative form, used when speaking to:

• one person you address with ты
• often a child
• sometimes a general reader/listener in short warning language

Compare:

• Не трогай сосульки. = Don’t touch the icicles. (to one person / informal)
• Не трогайте сосульки. = Don’t touch the icicles. (to several people, or politely to one person)

So this sentence sounds like a direct warning to one person, perhaps a child.

Because the warning is about icicles in general.

• сосульки = icicles
• не трогай сосульки = don’t touch icicles / don’t touch the icicles

Russian often uses the plural like this for a general class of dangerous things. It does not have to mean a specific known set only; it can mean icicles as a type of object.

Also, after трогать, the direct object goes in the accusative case. For plural inanimate nouns, the accusative looks like the nominative:

• nominative plural: сосульки
• accusative plural: сосульки

So the form stays the same.

Because the logic is:

• Don’t touch the icicles,
• because even one icicle may fall.

Russian uses одна сосулька to emphasize that just a single one is enough to be dangerous. It is a very natural way to express that idea.

So the plural gives the general warning, and the singular with одна highlights the minimum dangerous case.

Может is the 3rd person singular of мочь = can / may / might / be able to.

Here it means something like:

• may fall
• might fall
• could fall

So:

• одна сосулька может упасть = one icicle may/might/could fall

This is about possibility, not permission.

Because упасть is the perfective verb, referring to a single completed event: to fall.

• падать = to be falling / to fall repeatedly / to fall in general
• упасть = to fall (once, as a complete event)

In this warning, the speaker means a specific possible event:

• an icicle may fall down

So может упасть is the natural choice.

Compare:

• Сосульки часто падают зимой. = Icicles often fall in winter. (general repeated action)
• Сосулька может упасть. = An icicle may fall. (one possible event)

Because Russian distinguishes between:

• movement onto a place → на + accusative
• location on a place → на + prepositional

Here the icicle is falling onto the sidewalk, so Russian uses the accusative:

• на тротуар = onto the sidewalk

Compare:

• Сосулька упала на тротуар. = The icicle fell onto the sidewalk.
• Сосулька лежит на тротуаре. = The icicle is lying on the sidewalk.

So the case changes depending on whether there is motion toward the place or location at the place.

Прямо here means directly / right.

So:

• прямо на тротуар = directly onto the sidewalk / right onto the sidewalk

It adds emphasis: the icicle could fall straight onto the place where people walk.

In other contexts, прямо can also mean:

• straight (direction)
• exactly
• right (for emphasis)

But here the sense is directly.

The colon introduces the reason or explanation for the warning.

So the structure is:

• Не трогай сосульки: = Don’t touch the icicles:
• одна сосулька может упасть прямо на тротуар. = one icicle may fall directly onto the sidewalk.

It is similar to English punctuation in a sentence like:

• Don’t touch the icicles: one could fall onto the sidewalk.

Russian often uses a colon when the second part explains or justifies the first part.

Yes, сосулька is feminine.

You can tell because:

• it ends in -а, which is very often feminine
• the word одна is the feminine form of one

Compare:

• masculine: один стол
• feminine: одна сосулька
• neuter: одно окно

So одна сосулька clearly shows that сосулька is feminine.

Yes. Although the basic meaning is don’t touch the icicles, трогать can also suggest:

• don’t handle them
• don’t mess with them
• don’t interfere with them

In a safety warning, it can feel slightly broader than just physical contact with a fingertip. It suggests: leave them alone; don’t do anything with them, because that could be dangerous.