Plasm Capture — ruling 1

If the countered spell had {X} in its mana cost, include the value chosen for that X when determining how much mana to produce. For example, if you counter a spell that costs {X}{R} with X equal to 5, you'll produce six mana.

Plasm Capture — ruling 2

Your precombat main phase is the main phase that happens immediately after your draw step, not any other main phase.

Plasm Capture : règles, fonctionnement et interactions (MTG)

Plasm Capture is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Counter target spell. At the beginning of your next first main phase, add X mana in any combination of colors, where X is that spell's mana value.

How it works : Plasm Capture

Original English text — Wizards of the Coast / Scryfall

#1April 15, 2013· Wizards of the Coast
If the countered spell had in its mana cost, include the value chosen for that X when determining how much mana to produce. For example, if you counter a spell that costs with X equal to 5, you'll produce six mana.
No other card is named in this ruling.
#2April 15, 2013· Wizards of the Coast
Your precombat main phase is the main phase that happens immediately after your draw step, not any other main phase.
No other card is named in this ruling.